Player controlled survival mode creep attack?
I think its fun because the game will be perfectly balanced, fair game, everyone has equal chance to win.
If the player quits who sends creeps, a dead player or spectator could take his place. If no player took his place, it would continue with the normal surival mode attack algorithm.
The survival mode is quite boring because you can guess very well how it plays, so you dont need to defend very intellectually.