i’d majorly overhaul the gameplay.
i’d make each class fee like totally different games (warriors would feel like brutal legend. mages like infamous 2, etc), basically, i’d make the classes feel more action-based. less building up to one big hit like we have now with certain classes.
i’d make enemies/npcs feel less like a walking hitbox/healthbar, give mobs more than 2-4 abilities and make’em actually try to dodge our abilities.
speaking of dodging, i’d add the ability to dodge. little annoying how some class can dodge, and others can’t.
i would make mounts feel more like the vehicle system we have (with like, their own abilities/healthbars) that when they run out of health, you’d have to get them resurrected/repaired at a vendor (except in bgs/arenas).
i’d add-in player housing (cause folks have wanted it for years) with a little farm, similar to the garrison, but less ‘the expansion is based around this!’ and more just there for fun.
add companion characters (like the legion bodyguards) who you can romance and have kids with (that… might be more of a crazy idea than anything though) that when you sleep in your player-home with your family, you get a buff (like in skyrim).
a battle-pet system, where your battle-pet’s on your team feel like pets, with hunger and fun meters you need to refill every couple days, by feeding and playing with them (like with one of the battle-pet toys).
i’d add enviromental debuffs (such as desert heat that slows), and special armors, that are stronger in those areas and counter the debuff (like, a desert robe that makes fire spells stronger, and removes the heat debuff).
i’d add an all-levels version of bgs that require special pvp-armor that ‘mostly’ equalizes stats (just so 60s don’t one-shot everyone there), and would give different buffs based on your level, (like, a 20 paladin would have a VERY improved templar’s verdict, while a 40 would have improved judgement).
i’d make class-halls a bit more intrinsic to the game. like your class order would be their own faction, and they’d give you rep and rewards when you complete zone stories (and when you do all the quests in a zone). that way, you’d actually have to earn your place as their leader, and not just be in-charge cause someone said you were.
i’d add tintable armor. not remove the armors we have or change them! just… add new ones that can be tinted.
also, some more mounts with customization like the paintable mechanocats. that was a neat feature, little clunky, but neat.
i’d make end-game not be the whole game.
basically, i’d make the older content, just as important as the newer content by making chromie-time give access to old raids, with rewards that scale to new content. also, make one or two of those old raids, have armor buffs that are just-as-good for current raids as the new stuff (bad-example: magic resist on ahn’qiraj gear that’s useful against one of the new bosses, that sorta thing).
also, basically, if i added a system, it’d be something tangible. something that’d either be optional, or change gameplay.
there’s probably more i’d add, but this is all i can think of off the top of my head. but hey, that’s just what i’d do. (though, i might be over-thinking/over-reaching).