What I'd personally like to see happen:
Gateway removed. It was an experiment and, IMO, didn't work out. It feels clunky and poorly implemented, it's pathing is horrible and in PvP it's just downright awkward. It also causes PvE raid design problems (hi Iron Qon) where it ruins interesting fight mechanics.
Soul Swap removed. Affliction's dots buffed to compensate. Malefic Grasp redesigned or removed in favor of an affliction-friendly cast time spell. In my own opinion Malefic Grasp feels horrible.
Demonology being given a cast time filler. Touch of Chaos not scaling with haste and making Kiljaeden's Cunning largely useless for Demo isn't cool. It also just don't feel right.
Dark Soul nerfed across the board for all specs, and warlocks being brought back to the more "consistent, non-burst" style they had pre-cata.