My regret is over scoping and at the end only being able to make just one level for our game. It wouldn't have been a problem if it were any other game but this game's core mechanic revolves around punishing the player for over-extending in the earlier levels since the spells can only be used once.
(It's probably easier to understand what I mean by playing the game: https://itch.io/jam/gmtk-2019/rate/461352 )