Well, here's the big issue I currently have: The way to edit the level is that you can just move it around. You can't actually add a new Tile.
Now, my suggestion: Allow adding tiles.
You might ask: "How will the GUI work"? Well, it can work really simply.
What we can do is either have Keyboard shortcuts like Super Mario Remaker, or do like SMM and have it on the top, and have a dropdown of all the tiles. We can even do like the old Reggie style, and just have everything on the side.