Minion rush
My idea for a game mode is a minion rush style (no enemy players). Basically the entire point of the game mode would be to defend the base and not allow the minions to destroy the turrets for as long as possible. The best example I can give to something similar is Mann vs Machine from Tf2. You start out with so much gold and at the end of every round you are able to buy items. Maybe have some crazy stuff like jungle creeps pushing, waves of dragons, DOOM BOTS ..... the possibilities :o
I was also thinking about an additional talent tree when you start the game. Depending on what "spec" you want to play you fill out the talent tree accordingly (tank, damage dealer, support). Every time you win a round you would get a point to improve your overall effectiveness (having it build into the store as another window would probably be the easiest way to do this). Like for example if you were a support (the most difficult role to fit into this game mode I think), you could spend your points into giving your allies a chance to regenerate mana/energy/fury after killing a unit. The more points you put into this talent the higher the % chance. Maybe the old talent system would be sufficient but its just an idea :)
I don't think there should be minions for your team in this mode. This way tanks have a role in this game mode so everyone doesn't just play damage dealers and go crazy.
I have been thinking about this for a very long time, I think it would be so much fun to play this game from a completely different perspective.