My top three wanted classes ranked by likelihood:
1. Tinker
2. Bard
3. Shadow Hunter
Tinker has the most potential. One can easily see how a tank, healer, ranged dps and melee dps role would look. It has the most wide open design space thematically. The only odd overlap would be Engineering, but even then it would still have a lot of room. In terms of fun, I could see this being crazy good - little medic bots, sending explosive sheep, drones, etc.. all of the machines can visually change based on race choice, big mech suits, hitting shit with shredder blades.... lazers, rockets.... all you have to do is look at Mechagon, Motherlode and the Hearthstone expansions like Goblins vs Gnomes for a ton of ideas. After the Island Expedition guys and the random Tinker follower I thought for sure they were coming in 9.0.
Bard is such a cool RPG class that I really want to have one in Warcraft. Warcraft has some goofy bard stuff like the DMF bands, but nothing solid. We see a few NPCs playing harps, a couple flutes, but the foundation really isn't formed yet for a class like this... that makes me really interested to see how WoW would handle it. They exist in the MMORPG that WoW was mostly built off of - EverQuest, and they exist in many other MMOs like Final Fantasy and Rift. For all the problems with Rift... their Bard was actually pretty fun for it's era.
Shadow Hunter is a personal wish for me. It would be a little more challenging to fit this in between Shaman, Hunter and Warlock, but I always liked Vol'jin, Troll lore and that mix of hunter + voodoo + shadow magic + archery and daggers. I'm more and more convinced my wishlist of the other WC3 units I want like Dark Ranger, Blademaster, Necromancer, etc would be closer to a class skin or something... but it's possible we can see something special for Shadow Hunter.
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Enchanter is an interesting one. There are other MMOs that have something similar, like Mesmer in GW2, but mechanically I'm not sure how well something like this fits into modern WoW. Enchanter was all about CC and control, and you could throw some damage spells but really all of your power was in making sure your group didn't die from too many mobs.
In 2020 World of Warcraft where its all round them up and AOE them down, with boss fights where it's just minimal adds, I'm not sure how this translates.
Maybe if they brought back more CC mechanics or allowed them to play a buffer / support role where they would mana battery for people or get some really good single haste buffs they needed to refresh throughout fights.