I'm out of Ideas, maybe some new tiles could be added, or new features like multiplayer.
Random idea, no idea if it is helpful, but maybe you could add some sort of hotkey system? Maybe numbers for different blocks, and press the same number again to cycle through those blocks? Also, maybe adding hotkeys to switch between modes? Also, a selection tool for drag, copy, and paste would be really helpful. Also, the saving subassembly system seems a bit broken?