might be off in the future but something that Gravity Sketch doesn't do easily is Boolean subtractions - if you could do that alone with your protobash app that would really sell - there are lots of "pieces" out there that can be pulled into ProtoBash and then simple stuff like making a hole in an object while you are "holding" it in your hand and then exporting back out as an FBX or OBJ without messing up the UV map would be such a win - good luck and God Bless
