If I would guess what Valve would pull of it's hat (pun intended) I'd say it's a new game mode. Payload really shaped TF2 to what it is today and if they create something as popular it could greatly enhance the game longevity.

But being Valve, anything can do. I won't surprise if it trolled us all with some crowbar unlockable so people can smash open crates or something.

BONUS - Free DOTA2 comic