I'd love to see:
more abilities
more dungeons/quests in existing dungeons
an increased level cap
improved co-op enabling saved online characters, mainly because at the moment there seems to be little point in playing co-op apart from getting achievements
I'm no programmer, but I'd guess that creating new dungeons using exisitng assets wouldn't be too difficult for the devs.
Usually, I don't cry out for DLC, but this game really needs a few extra hours gameplay. It's fantastic!