Going down the list, starting with plate…
Warrior: Gladiator spec should return. Shield-using DPS spec. Maybe give them a unique ability to use polearms one-handed with a shield? Just polearms, not other two-handed weapons. For the whole, Roman centurion kind of feel.
Paladin: This one is kind of hard, but I think maybe a DPS caster spec? Big fancy lashes of holy light, that kind of stuff.
Death Knight: Bear with me here: Healer spec, Fleshcrafter. Rather than “heal” people, what you’re really doing is using necromancy to bind their dying flesh together.
Hunter: …I dunno! They’re hard. Feels like we’ve covered all the angles already. Only thing I can think of is some kind of weird spec based around fusing with your pet to do stuff and get like, bear claws or whatever, but that’s kind of wacky.
Shaman: Something Earth-based. Probably a tank spec? But I’d be fine with it being DPS or whatever. But Earth feels like the least-used element. Should be a spec focusing on it.
Rogue: Sniper! A ranged DPS spec that has the particular flavor of “fighting dirty” of just…shooting you from really far away. This would probably shake out weird in balancing, though…
Monk: Some kind of “Brawler” spec. Probably DPS, maybe tank. No kung fu, no fancy stuff - just pure street brawling.
Demon Hunter: Some kind of caster spec? I was thinking maybe base it off of Illidan’s thing with throwing down his glaives and making fire spirits come out of them. Maybe something like that? You throw down your glaives to act as “turrets” and shoot abilities out of them, or something along those lines. Probably heavy on glaive positioning.
Druid: These guys feel “done” to me, but if you had to add a new one, I’m thinking maybe something based around insects/disease? That’s a part of nature too.
Priest: These guys are tough, but I think maybe a melee spec, Shadow-based. Based around giving into the Void and becoming a shrieking lunatic who rips things apart with your hands and shredding shadow juju. Maybe give it limited stealth, based not around being sneaky, but around messing with peoples’ minds so they can’t see you? I dunno.
Mage: These guys have lots of options. Chronomancy is valid, but I personally would like a “battle mage” spec. Blood Mage is also an idea with some merit.
Warlock: You remember how in Legion there were guys called “Fel Knights”? Like Fel Paladins? That. I want that. There’d be no giving us plate armor probably, but Warlocks can already use swords - why not expand that into a melee spec? Sword in main hand, offhand item in offhand, does melee felstuff. ALTERNATIVELY, you could probably swing Blood Mage as a warlock spec too. That’d be cool as well.
…I’ve thought about this for a while. It’ll never happen! But it’d be cool.