New content, across the boards. Missions/features in missions, upgraded graphics, maybe voice acting. New powers, maybe gear of some sort/more combat or utility powers, or vehicles for travel powers. More incarnate stuff.
New things to do besides grinding missions and influence. Working the AH is okay, but kind of boring. Maybe things like zones that need constant maintenance and help from heroes or they degrade (Faultline, Boomtoom, etc) Make it feel like you're actually doing something that affects the game.
And if we're shooting the moon, fix PvP. It's a mess as it is right now.
Also, personally I don't mind paying for subscriptions. I realize not everyone can, and it's free now, but an optional subscription like they had before was awesome.