Sounds good. I'm guessing you'll be able to place objects using abilities? Or will it be like you select the box tool (unit) and when you order it to move a box will be created there? You should make a lower/raise ground tool too, that'd be pretty cool. An apply force tool would be good if possible - maybe it would shoot the target object away from the camera? The problem is you can't use that function in multiplayer.
Sounds like a promising map all in all, but I have some doubts about the possibility of doing it for 12 players.