Nice and simple. Like all good puzzles.
For a new mecanic, you could add modifiers on the floor like +1 or -1, when a piece goes to one of these it adds a +1 or -1 to the piece, in addition of (eventually) the -1 coming from the move.
This one is a crazy idea : you could add an action to explode and repulse nearby pieces.
But I guess there is much more interresting puzzles with just the mecanic you have.