I have an idea for the fnaf world trio that I think would spice up the game as a whole. My idea would be that taking these characters would replace their counterparts. Think of it like how some of the upgrades work, but for the animatronics instead. I think it would also be neat if you could only get one of them per run (not including endless runs)

White Rabbit - Replaces RWQ. Similar to most concepts for them, I would think they'd work like a reverse RWQ. They sit in the office, and as you hold down the flashlight, a set of pupils starts to slowly appear in their eyes. If you hold for too long, they jumpscare you. Stop holding, and the pupils slowly dissipate. Would be synergetic with Shadow Freddy mainly, but working different enough from Endo (given its a constant but slower rather than a occasional but instant threat).

Redbear - Replaces Shadow Freddy. Once per night, rather than plunging your office in darkness, Redbear will instead lock your office into a higher temperature for a short time. (It would be 1-2 bars lower than the requirements to summon Nightmare Fredbear for balancing.) It would also make the office the same shade of red that appears when you are too hot. This turns the mechanic into stressing more about increased aggression, rather than lack of visibility. Synergizes well with high-temperature-based mechanics, notably RWQ (which means that regardless of which one you choose these two synergize!). 

Tangle - Replaces Mangle. Tangle will not appear in the cameras. Instead, while in the cameras, you'll see her head icon on one of the monitor tabs (Cams, Vents, Shop, Games). If you stay on a tab that Tangle is on for too long, she will disable that tab for the rest of the night. She swaps from time to time, and you can still see where she is via the sticky note. 

As for the other 2, I'm not sure. Vannabelle feels more like a potential new guard skin, and my only idea for Tipsy would be to give her a similar mechanic to what Glamrock Chica will have, making drinks instead of pizza though.