This is a great way of applying the "only one" concept to an existing idea! Really cute graphics, quite clean.
Have you seen the GMTK video on puzzle design?: https://www.youtube.com/watch?v=zsjC6fa_YBg&t=2s
I'd tried making puzzle games for years and could always come up with neat mechanics but I always fell flat when it came to making levels that were challenging but didn't feel cheap. The advice in that video helped a ton.
If you'd like to see how our team applied those lessons for this jam check out our game, Ka-Bob: https://itch.io/jam/gmtk-2019/rate/460345.