New content on a regular basis. Content, here, meaning costume sets, new mission arcs at a variety of level ranges, zones, enemy groups, powersets and archetypes. Would also like to see Mission Architect updated because it feels really, really outdated and clumsy to use at this point. The interface takes forever to navigate and things pop open and closed in weird ways, making it hard to see what you have and haven't done. Some sort of out-of-game editor for it would be nice.
Also the ability to use NPC costume parts on player characters. Or force 'invalid' costumes to load into the game. I know we can do stuff like use Titan Weapons melee weapons on Staff Fighting characters if the game would just let us force the costume settings instead of throwing the error message at us. There's more sliders, like the Arm Size one that also are functional and visible to other players when used but the game tries to reset whenever you log out. This stuff should be made usable, maybe in some sort of advanced costume editor mode?