Basically take a Normal FNaFb Concept and basically add one (or more) of the following Ideas to a side mode:

More Dangers and Debauchery: This is adding a whole lot of weird enemies that really don't have to do with the place its in. You could also hide bosses in different areas (I.e., Vending Machine, Rotten Pizza, Wallmasters, Basically anything that's original)

Chance Time!: Whenever you wanna buy something from a store, It'll have a random selection of items per run. Chests will give a random item. Enemies have more than just 1 skill, and there could be more than 1 type of enemy (i.e., Party Hat alphas being either a Healer or a "Tank".)

Impossible Daredevil Run: You can't save, have health degen, all healing items heal very little, can't level up. Good luck.

Yeah that's about it.