I would use new assets. I checked out Cubin, and the game mechanics were good, but there were no animations. Jams are a great place to try new things. I would like to see everyone try doing something new.
As in your case, I would focus on aesthetics, now that you have a good character controller. I would love to see your game have a character as the player. Pixel art is always a good option, also because Gdevelop has a pixel perfect feature.