Some of the missions are already like that (like the stage where you have to find the biggest cow (I sucked at that, I kept picking up the spotted traffic cones or a carton of milk), or pick up crowns). You can play 2 player arena battles where if your katamari is bigger than the other person's, you can roll them up.
Still more replay value would have been nice, something like being able to go back with random objectives or randomized worlds.
They're working on #2, so hopefully they'll make the game bigger if not more-replayable.