I've only had a quick game but liking what I see so far. Reminds me of a cross between the game Edge by mobigames and a form of 3D Tetris. Well done and thankyou. My son is into Rubiks cubes so will likely also enjoy this one.
Maybe a easy mode would good, but the idea of a challenge is good too. He is only 10, but very good at solving a Rubiks cube so has a good mind for spatial puzzle solving, I think he wanted to get back to playing Sea of thieves so Subcube could only hold his attention for so long... haha.
One suggestion (and I could be missing something that already exists in the game as I have only played it a couple of times so far), at the start of the level it tells/shows you what you need to create, however during a longer level you may want to double check what the goal is so it would be nice to be able to show the goal again in case the player forgets. This could also be useful for when a child is playing and they call on a parent to help out midway through the level, it would be good for a parent to see the goal if they missed it at the start. One method would be to press a key to bring up the goal overlay again or have a smaller version of the goal visible in a corner of the screen at all times.
Thanks again for a great gift.