A rogue-like dungeon crawler mode?
Okay, for those unfamiliar with rogue-like games. Basically you go into a multi floored dungeon. The maps are randomly generated mazes. The goal is to get to the final floor without dying. If you die, you get sent back to the top floor and lose all your exp and items. I think that would go perfectly with this game.
Basically you start out at the shop with some cash. You go down into the dungeon, Some floors will have bosses (like actual bosses that require you to out maneuver attacks), not baron nashor and drake who stay in the same place repeating the same attacks. Recall will have a cooldown, but allow you to go back to shop then go back to where you recalled from. Instead of normal trinket, you get a resurrection stone to resurrect fallen allies (either channels, or is out of combat only)
http://en.wikipedia.org/wiki/Roguelike
There could even be minion spawn spots that you can capture to help you push towers and other objects to progress lvl.