This sounds just like (can't remember the map name >.<) but it had the exact same idea. Each player had one hero which he/she could upgrade and buy stuff for, there would be one "commanding" player which anyone could be (he/she selected which way the troops would attack along). The map also included an insane AI which allowed you to play the map in singleplayer mode (it was very fun!).
If you can find out what map it is (still can't remember...) I suggest taking a look at it to get some more ideas for this map.