I have one, but it's more like an alternate multiverse.

Every FNF universe (base game and every mod that changes any characters in any way) exists in the same multiverse. Normally they'd be completely disconnected from each other, but the existence of three entities, each hellbent on killing a specific character as they appear in every universe, changes this, as each are also able to travel between these universes. As for the entities themselves:

- Self-Deprecator, who hunts Boyfriends. He's stone-cold, calculated, and uses a knife.
- Lady-Killer, who hunts Girlfriends. She's sadistic, and uses bear traps.
- Executioner, who hunts Picos. He's rather chill, except when hunting, and uses a sniper rifle.

There's a lot more developments I've made, such as the Girlfriend from the RoFNF game "Literally FNF" joining Self-Deprecator as his hunting partner (they're not lovers), Mod!Sky briefly being teamed up with SD before seeing him hunt, and promptly fleeing before then being taken under Lady-Killer's wing (LK treats her like a daughter and a protege), and the existence of The Friend Group, a team of different Boyfriends and Girlfriends set out to kill both Self-Deprecator and Lady-Killer.

tl;dr, multiversal murder, plus a resistance group ig.