A large (5m x 5m) square platform made from sandbags and wood top that can be built on top of and is blast resistant (can only be taken out with TNT). Cannot be built indoors. Limit is 6. This would be useful for a number of different things. Can be used to elevate a position, protect from bombardments, fill in trenches meant to trap tanks, combine a couple together on top of a trench to make a player-made bunker, etc.
I think this could greatly help both offense and defense significantly, and make engineer squads more viable again.