Some suggestions i would like to see - mainly based off of the good old expansion (why fix something that isn't broken)
As Flayz says - balance 1v1s. Dueling is just a fun aspect of the game that have been completely ruined. Give some titles back in 2s and 5s. Makes for a much more interesting competitive arena environment. Remove the fucking garrison shit. WoW is all about the multiplayer factor. Who the fuck wants to hang around alone in their house in a game (WTF).
Remove HCs and number of people scaling. PvE was fun when, lets say u killed Illidan, u fucking killed Illidan. Not some easy or hard version of him. That leads to my next suggestion. Remove transmogging. When people had the cool BT gear, you knew they were good PvEers. There should only be one version of gear from one raid/boss. Makes the game much more "realistic". Same goes for PvP gear. You should be able to get better/cooler sets/weps with higher rating. There should be some more advantages in being a skilled pvp player. Stop the whining about its not fair for people below 2,2 or 2.4 - fucking get better or wear ugly ass gear - doesn't have to have a lot better stats. Just so its worth getting and enchanting and looks cooler.