The ability to add/replace things like: Food items, buildings/bushes, music/sounds/levels for rhythm game, tilesets for the map, earthling and character graphics, etc.
Even if this would take too long to be implemented having a workshop or at least being able to mod the game in some aspects could open the game with tons of new possibilities as well as adding a lot more replayability than it already has.
If this can be implemented then i would patiently wait years for this feature to come out if it does (which i hope it doesn't take actual years but you guys can take your time if you have to)
@Clox: Unfortunately, it would probably be a few years for this to happen IF at all. Unless we open a few Amped Jackpot presents magically before then
Unlikely to happen until 2.0 DLC.
Steamworkshop (mod support) would be great