A Sandbox Mission

I would love to eventually see a sandbox mission, similar to some other tower defense games.  The difference here is towers can only be placed on raised land, so this would make it difficult.  But I'd like to see some sort of mission where the whole map is flat.  The enemies could come from any point of the outer edge of the map and it's up to you the player to force them to one or more pinch points.  And place towers where ever.  

Maybe this could be done by allowing the player in the game to raise land to place towers, but at a cost.  Just a thought.