Unlocking of bonus levels
Currently when a level is solved, it unlocks both the next level, and a bonus level which is an harder version of the solved one
The bonus levels are very difficult for unexperienced players. Maybe it would make more sense to unblock them later: when completing a "chapter" or when completing the whole game. It will be more interesting for players to go back to a whole area they already solved, and do it again in an harder mode. It is not very interesting to solve a level, and immediately after, solve another similar but more difficult level