They need to make loot per-player (ala Diablo 3) rather than competitive - I'm actually surprised they didn't do this for BL2. And they need to secure it from hacking, preferably by implementing it via Steamworks to allow weapon trading outside of the game.
Oh, and I'd like to see some more variety in classes. I'm not impressed with the skill trees in BL2. Still, a sequel sounds like an obvious move.