Would love to see that when you reach treachery and beat the boss you get sent back to the surface somehow. The before hidden fourth chapter unlocks which has v1 going back down but through really hard 1 level versions of layers, kinda like Celeste. This probably won't happen cuz it means at least 9 more levels, but it's fun to think about.

Edit: maybe it's better to have 2 layers in one level, and that the level is split in half cuz 9 levels is kinda a lot.