Hi.
Do you have any examples of crazy secret/bonus levels (or rom hacks) that don't follow the theme/aesthetic of the rest of the game?
I want to incorporate something like that into my own game (and nobody can stop me)

Kero Blaster secret levels
MyHouse.WAD (recommend playing it for yourself if you haven't seen it yet)
Omori Blackspace (turns the game into a creepier Yume Nikki)
Sonic.exe

@Verb these are amazing examples! These all fall into the "creepy" category, although I'm looking for literally "weird" levels

Probaby the closest example I can think is some of the celeste post-game levels. I really like level 8,

Maybe something like Don't Eat The Mushroom (sound warning)?