The rules for Theorycrafting is that every build should have some Endgame in mind. This could be PvE (Trials) PvP (Emperor, and now Volundrung) or 100% Completion.
This is 100% Completion, but Tamriel is huge, and the map is getting filled in with new chapters like Elsweyr, Summerset, and Morrowind. This takes me back to The Elder Scrolls III: Morrowind. Where it all started for me, and how I fell in love with the series.
There were 2 classes that exist in both that game, and this game: Nightblade, and Sorcerer. (I'm going to do another 100% Stealth run with a Nightblade, too.) However, one of the most unique, and fun classes was called a Spellsword:
"Spellswords are spellcasting specialists trained to support Imperial troops in skirmish and in battle. Veteran spellswords are prized as mercenaries, and well-suited for careers as adventurers and soldiers-of-fortune. "
There's also a bunch of Skill Lines/Stats that roughly translate over to ESO, but the basic spirit is a Hybrid, Spell+Sword. Destruction requires a Staff, Conjuration doesn't. That's the main reason I went with Sorcerer (Also, there's no Stunted Magicka or Spell Absorption in ESO, and the Atronach actually grants you Magicka Recovery!)
Fortunately, The class and Race (Dunmer) also lend themselves to a Hybrid, that can use Spells, and Swords equally well. So, besides getting all the Skyshards, Lorebooks, beating all the Bosses, Dungeons, Trials, and Quests, they also at least Use all of the Skill Lines availible to them. (Meaning, they can't exactly use Ardent Flame, or Siphoning.) Also, all the Skill Points, so they have to spend them on something.
Here's how it works, the primary stats are going to be Weapon, and Spell Damage. As much as you can get, because this is a Hybrid, you can't be a Jack of All Trades without being Master of None. However, you can minmax to the point that All of them are Viable. Not OPtimal, Viable. You're not going to use a Bow as well as a dedicated Bosmer Archer, however that Bosmer Archer is not going to get as much out of a Destruction Staff as you.
Also, the Trinity has to be addressed, because to get 100% Completion, you have to finish the Trials, at least once. You're not going to Collect all the Sets, "You can't have everything, where would you put it?" But, you have to be good enough in A Role to finish the Trial, with a Group. So, let's start there:
This is the best role for a Hybrid, because a Tank can fulfil this role with Stamina, Magicka, and Health. The Healer, and MDPS is going to be pure Magicka, and they can if you keep the Aggro off of them. The SDPS is going to focus on Stamina, to get the most DPS out of their rotation, but a viable Tank has to balance between Stamina for Blocking, Magicka for Buffs, and Shields, Health to be Healed by the Healer. So, for the purpose of running Trials, that';s the bset Role for you to Play.
5xHeavy, 1 Light (Sash) and 1 Medium (Bracers) for Undaunted Mettle, and Maximum Armor. (Belt has the least Armor, Gloves the second least.) Ice Staff (Sash, and Jewelry) of the Prisoner's Rags, Sword, and Shield of Song of Lamae. (If you can get Monster Sets, great! I didn't stick around to farm the Lottery for those. I don't have time for that, when I have to get all the Skyshards from Cyrodiil, too. So, I farmed Coldharbor, and bought the Rings from Guild Stores, then crafted around them.)
Magicka Recovery: Okay, you know Dark Conversion? It converts Stamina into Magicka, is basically the ESO version of Sorcery (Instead of Spell Absorption.) The Prisoner's Rags generates Magicka from Sprinting. So, you can Morph it to Dark Deal to turn that Magicka into Health, and Stamina, however I don't use this in Combat. I use it to recover from Combat, and move to the next fight Quickly. (This contributes to the Squad getting a better Time.)
Also, Elemental Drain, and Energy Overload. Ultimate is another Resource, that a Sorcerer can convert into Magicka. Health too, but I'm not going to do that, especially as a Tank. A Tank has to keep his Health up, so channeling it for Magicka just makes the Healer work harder, your Shield weaker, and nerfs your Healing Received. Way to break a Tank. Let's just assume that Health is the one thing a Tank isn't going to Sacrifice for Magicka.
Resource Rotation: Dark Deal is bad for a Tank In Combat, because you have to drop your Shield, and stop generating/Holding Agro for the duration. It's just a second, but that second of Vulnerability leaves you open to CC, you'll have to use Stamina to Break Free, (Or cast something like Immovable Brute, when you're already low on Health, and Stamina.) It's not a good Deal, in Combat. You have a job to do, and you can't stop for a second to make up for mis-management or Resources.
Bar-swap: That's what the Ice Staff is for. When your Stamina is low, you can switch to the Ice Staff, and use Magicka to Block/Taunt/CC while your Stamina recovers. You don't have the Staminasteal provided by Elemental Drain, but cast that again, before you swap back to your Sword, and Shield. It's still active, so you can keep attacking that target, to continue getting Magicka back, from Stamina.
Bash Cancellation: Here's where Champion Points come in, Sprinter lowers the cost of Prisoner's Rags, and Bash Focus reduces the Stamina Cost of Bashing for Damage. Not much damage, you just have to Bash once every second, and it helps it that second Interrupts a Caster, or sets someone Off Balance stopping a Power Attack (Uppercut, or whatever.) You're not doing it to deal Damage, you're doing it to proc a tick of Elemental Drain, until your Magicka is back up.
Power Surge: One thing you can do, however is get Health back from dealing (Critical) Damage. The best spell for that is Hurricane, it's Best in Slot, not because of how powerful it is, but because of all the things you get out of that 1 Slot: Major Ward/Resolve, Minor Expedition (Again, Sprinting to the next Fight, and recovering Magicka) AoDoT that can proc Crits, to activate Power Surge. This is another one, that gives you 3 things for 1 spell: Major Sorcery/Brutality, and Health back from Crits. If you're a MDPS, you'd want Lightning Form, (And a Lightning Staff, but I'll get to that) if you're a SDPS, you'd want Critical Surge (And Dual Wield/Bow) but instead you're a Hybrid Tank. You want Major Brutality, AND Sorcery. Not more Health at the expense of one, or the other. (This also comes out when you respec for Hybrid DPS)
This is Solo. Not for a 4 man Group Dungeon, and not for a 12 man Trial, your DPS won't be high enough. (Which is why you spec for a Tank to sign up for Group Trials.) However, there are Solo Quests, you'll have to do to finish that Questline, and complete that part of the map. The most obvious is The Harborage, which I'm in no hurry to clear, because if you save the later ones for Level 50, all the rewards from it are CP160 when you receiver them. Same with the Mage's Guild, and Fighter's Guild. Save them to get Truly Superb Glyphs of Health/Stamina/Magicka.
That also means taking on the Ancient Clanfear, Gutstripper, Spinesnap, and Doshia. Not that these are particularly tough enemies, but there's some shadenfreude from utterly curbstomping them as a CP160+ Spellsword, after doing it countless times as a level 7 noob build.
Lightning Staff, and Dual Wield
I'm not going to bother with the Sets on this, because honestly, you don't need them. I've done it Naked, farming the Spell Scar when other players showed up to catch me wiping them out en-mass in a black bra, and panties. I even got comments on it, in chat! (I put a lot of work, hours into ever character to make them look just right. Especially my Role-Plays) If you have Alkosh, Chudan's, Master weapons, and Spriggans, use those. Sprigs is pretty easy to get, if not use Hunding's rage, the difference is negligible.
Passives (Stack) Energized (5% Shock, and Physical Damage) Ancient Knowledge (8% AoE damage.) Expert Mage (10% Weapon, and Spell Damage) Power Surge (20% Weapon/Spell Damage) Ruination (Weapon/Spell Damage) The Warrior (Weapon Damage) and Agility (15% Weapon Damage.)
Now, you'll notice those last 2 focus on Weapon Damage, and Agility requires 5 piece Medium Armor. If you have it, Draugr Hulk, if you don't Hunding's rage works fine, but that's because the Destruction Staff is tons more powerful. Stark naked, than Dual Wield is. (You don't get the 8% buff to AoE from Ancient Knowledge, but there's a lot more to it than that. That's the biggest dent, though.) Also, like the Tank spec, Stamina Recovery lags behind Magicka Recovery, particularly because Stamina is used to Block, Bash, Sprint, Roll, and cast Feats. That's why I tested it Stark Naked in Craglorn (Group areas, so they're scaled like Dungeon mobs) Destruction/Sorcery is just stronger, so the Swords need the boost to Weapon Damage to Balance the Spell/Sword. You will get better performance putting it all into 2 staffs (Inferno Staff for Single target, like a Boss.) The strategy isn't Optimizing 1 thing, but being Viable in all things (And master of none) for Completion.
Expert Mage: Here's where the greatest single bonus comes from, and incidentally the reason why Spell Damage>Weapon Damage on this build (In this patch, and why Magsorks are making a Killing in Cyrodiil right now.) 2% more damage For Each Class Skill slotted. On that bar, the Lightning Staff/AoE bar. That's your maximum damage, and for a pure DPS, that's what you want to minmax for.
Lightning Flood: I went for Maximum Radius instead of uration, because that fits in the maximum number of targets. Also the Maximum Diameter means I can cast it farther from my toes for the mobs to run through to get me.
Streak! And get Stunned, in the AoE, then turn around, and run back through the Lightning Flood, to try to get me.
Hurricane: Again. Same purposes as in the Tank spec, only this time the Damage is more significant than the Damage Mitigation.
Quick Cloak: (Swap Cast) You don't want to be off of your Main Bar too long. Honestly, that's all I use in Duel Wield for the Duration (15 seconds) but also Major Expedition, and AoE Resistance, all in 1 slot.
Power Surge: Again, no reason this needs to be on the Main Bar, because it doesn't deal damage, it just boosts it, and Heals from the constant procs.
Impulse: Spammable. That's all it's there for, if I don't need to swap/cast my Buffs, then I don't. I just throw some more Shock Damage in the AoE.
Whirling Blades: Execute. Mass Execute. With Quick Cloak, and Hurricane up, I don't have to use Streak, nor Sprint. I don't want them Stunned, I want them pissed off, and following me. Solo, the only one they're going to attack is me anyway, until the PuGs show up, and catch me playing with my food in a bra and panties. (Tattoos, Morag Tong Spatter Lenses...) Like Impulse, it's also a burn off extra calories before I switch back to laying down the burn, because there's no reason to let that Stamina go to waste. All the Stamina Recovery is useless if your green bar is full.
So yeah, that's basically it. Spin to Win, but also playing Tesla Tower in between. Anyone who's played Spin to Win knows the Farming Run. Only instead of pulling a bow to drop Endless Hail/Caltrops, it's a Lightning Staff for Hurricane, Liquid Lightning, Energy Overload (Spamweave, don't hold down the "Unlimited Powah!" button, tap off shots in between Impulses to keep your Magicka up.) Depending on how many you Pulled, you might run out of Magicka/Ultimate, but by then, your Spamina will be back to full, to finish them off with Whirling Blades.
Critical: Okay, this stat is surprisingly low, because Minmax. I just have to add that in here, you can't focus on Max Magicka, Magicka Recovery, Spell Damage, Spell Penetration, Spell Critical, AND Critical Damage as a Pure Magicka build. Not and have any 1 Maxed. If you wear Necropotence, you have to give up all those other Spell Stats, and 1 Active Slots, just to keep a pet up full time. (Or, run a 1 bar build, and give up Half of your Active Slots. I miss that Overload Bar.) As a Spell/Sword hybrid, something's got to give. Everything goes into
Traits: The usual suspects, I went Invigorating and Absorb Stamina glyphs on the Swords, because that's what I'm lacking the most. Incidentally, the flex slots on that back bar are for Overland, so is the Jewelry set/Lightning Staff (Prisoner's Rags) Just like the Tank, Sprint for Magicka return, and Dark Deal for Stamina recover between fights, to get to the next Quest/Skyshard/Dungeon/Fight that much faster. You're never going to get 100% without some speed boosts, and I already did it once with an Orc. Now, I'm doing it again with a Dunmer Hybrid, to test the build out on All Content.
However, no 1 build works for all content, without some adjustments. That's why you spend all those Skill Points on all of the Weapon Skills. Not because of Completion, but because you need to be versatile enough to Put around Kenarthi's Roost/Bleakrock/Stros M'kai/Betnikh, drive across Cyrodiil (And counter-gank when someone inevitably catches you) and sink a hole in 1 soloing that world boss that's scaled for groups in Craglorn, or the Maelstrom Arena, or...
You don't have to go it alone, and fortunately, Pets count toward Expert Mage. So, you basically just need 1 skill from each Weapon (You're using) to keep that 10% bonus. The problem is, they take up 2 slots, each (On both bars) so that leaves you with 2 Spells, 1 Weapon skill, and 2 Utlimates.
Volatile Familiar, and Twilight Matriarch fit in best with the Lightning Staff for VF's AoE, and Twila's heals. Plural, early on Clanfear is great for healing you, and him but also Twila tends to stay back. Zap them from a distance until you call on her to Heal up the Party.
Lighting Flood, Streak, and Elemental Drain: These basically keep you Protected, DPS, and Debuff 1 Target. Speaking of which:
Power Surge, Critical Rush, Daedric Prey, and Endless Fury. I tested Executioner, Reverse Slice, Wrecking Blow, and all that. Heroic Slash, Brawler/Carve, Momentum Vigor... I playtested it, stark naked, in Craglorn. (It's the biggest sandbox, with the most variety, and best profit from Drops to spend on Respecs.) I don't need Major Heroism, I can keep up Aggressive Warhorn without it. (As a trials tank, that's All you need Ultimate for, any extra is Gravy, I can bleed off with Energy Overload.) Then, I put my Dreamer's set back on to spamweave Streak all the way back to the Wayshrine to respec, and try another combination.
This isn't theorycrafted, it's tested, and it works for me. The way I Tank, and the way I solo for fun. Also, the fastest way to see All of Tamriel (Including DLCs) in a 1 month subscription, if you have as much free time as me. Solo, so I don't have to wait for my group to logon, the LFG to match make, or the damned Dragon Knight to catch up. (Sorry. They're slow. They are, moving on...)
However, there's no Flex-slots in this build, because they are, ALL Flex Slots. That's what this build is all about, when you get t the 7th hole, you can drop your golf bag, and get out your own 6 Wood, because that's what you need here, and you don't have a Squire named Caddy. This means more runs back to the Bank, the Crafting Stations to break down your Haul, and the Respec Shrines to respec. The Mundus Stones to replace the Lady with the Steed, Again, and get back to the quest at hand in Record Time, because that's part of the build too.
And fun, it's fun. For me, your mileage will vary, but there's enough (12) flex slots here that when you do get that Monster Helm, you can fit it in (And craft around it.) When you get to that part of Elseweyr you've never been, and face an enemy you've never seen before, even if you wipe the first time, you know you have the tool for the job in here someweyr...