Wow simply can never be balanced. It would have to became "the world of one class" and even then there's lag and ms that makes it unbalanced. If you say otherwise I'd like you to show me the math for dmg vs range vs cc capability vs resources vs etc. Yes, it can get close to "balanced" and that would mean making every class and spec avaible -> pretty close to being godlike. Even then what avaible means? Good in arenas? bgs? worldpvp? Sounds like making a bike that can fly, swim and shoot.
As far as World pvp goes, it relies on geared PvPers ganking "Newbies" who never wanted to join the pvp (remove flying comments and such stuff).
Personally I think they should make "Total pvp servers" which you could server-hop to and it would include no flying, but better rewards for pvp (world pvp titles / skins) achievements while playing and killing on those servers. That way casual players don't have to deal with 12m hp tanks buffed with weird stuff and pvpers could match pvp people / guilds in large open island pvp called "Broken isles" and be rewarded for it.
ps: for those who say Wotlk was the most balanced, pretty few specs were dead or nonexistent.