I think the UT3 engine would be nice for a AoS style map. With a little bit of Unreal(UC) and flavour. Could also put all these custom characters to the test. If the Demo-Guy doesn't show up...
2 Bases, creeps, Towers/Obstacles, Common goal, 40-60 min games, Teamwork and a whole lot of characters each with their own personal touch.
So simple, yet so brilliant.