Basic idea: so night 6 boss fights are cool. why not make a full game about them? You have to go through a gauntlet of different boss fights in different offices (themed around older fnaf fangames, because why not?)

Structure: There's a line for you to cross, each stage containing its own boss fight. You have lives in place, so that you don't have to worry about starting from the beginning every time you lose. Plus, there's a shop for you to earn items, like giving you extra power or something to give you a bit of an edge. Also, all rushes are randomized in terms of bosses so no two runs are the same.

Bosses themselves: Now I know old fnaf fangames didnt quite have concepts of boss fights yet, but I think itd be fun to theme them nonetheless. Maybe they feature an easter egg character, or a character in the spirit of the old fangame, idk. I also imagined that the offices could be randomized to change up the mechanics, but thats difficult to think of.

Other distractions: The bosses won't be alone, obviously. After you beat the first stage, other enemies will begin assisting the boss in killing you. To put it in perspective:

Round 1 - no distractions

Round 2 - Withered Chica

Round 3 - Withered Chica and the face

I'm not sure if they'd be able to take away lives from you, but I know that theyd at least trip you up in some way if you ignore them. To make them a little less annoying, once you finish a round (winning or losing) with them in it, they'll be added to the bestiary where you can learn exactly how they function. Bosses don't get bestiary unless you beat them at least once, however.

I am very tired so idk if this made any sense buuuuuut this idea tickles my brain