You know, the problem with world of warcraft is that the developers can't make content faster than the players can finish it. Also of course there is the lack of complexity and challenge in the end game.
I feel like both problems could be solved if blizzard implemented a removal of levels alltogether. Now bear with me. The way in this would work is to kind of level the playing field. The problem with world of warcraft is that you have a level of avatar strength even god has never seen. So if they were to squish that with this that would reverse the effect where content is useless because the enemies are too easy. Sure, they would have to give another reason for players to go and kill enemies they never would. Perhaps a retooling of the reputation/pvp/faction systems where your grinding has an actual effect.
Grinding was always a part of mmo's, so why not turn the grind around into the main focus of the game. Which would remove the grind, by making it fun and challenging.
It would be great if they could employ this to create a level of challenge reminiscent of demon's/dark souls, eve online, skyrim with requiem etc.
If it works in the way I envision it, there would be a great amount of players actually playing around in every zone where there are formidable enemies. Not necessarily enemies formidable by the standards of their stats, rather, the enemies would all probably have to be retooled. For example, a rat in pandaria can be level 86 or something and this would change. Suddenly the entirety of outland is like an instance. You can't actually mess around there without preperation and a party.
Of course, this would put more complexities down on the players, and they can't lone wolf it like they want to. So perhaps this would improve the game and lose blizzard their millions, but I don't know.
Really this isn't what HAS to happen, this is just my one idea to bring challenge, complexity, actual systems and player cooperation into the game. Because it's very strange that you can't just play an mmo like a singleplayer game, it's basically the preferred playstyle. You should force people to work together. Ah but whatever, Blizzard will just poop out more useless content and people will love it.