-Make it possible to LOAD maps on single player mode.
-Add the function to export/import maps, so that players can share their creations.
-Make the building system as simple and efficient as Cubic Sandbox, so that there is no space between the walls. That can spare a lot of work, reduce the amount of props in the map and improve the appearance of buildings.
-Add more wall formats and sizes (image1: some suggestions).
-Add more stairs, with different directions.
-(Image2) Add a search function in the inventory, to find props easily (it's not so important now, but if you add a lot of props just like in Cubic Sandbox it will definitely be useful).