Very nice puzzle mechanic: simple and effective.
With some polish, this would be fantastic. Just throwing a few ideas at you:
- I think many players would appreciate some useful buttons/keys (e.g. "reset", "undo". "redo")
- Good tutorial levels and no instructions needed! Though, might be good to have a few hints that would pop-up for players who are completely lost.
- Take some learning, but it's possible to have fairly direct controls over the characters. It seems that more difficult and/or complex levels would need to be tightly designed. I can imagine them being very (perhaps frustratingly) fun.
- With such a simple core mechanic, there are many variations (e.g. changing the shape the box, additional type of characters, etc.) for players to explore. That said, I feel like there's already a lot of potential with what's there now.