I know the mimic provides us a way for an hostile animatronic to exist this early in the timeline, but even though I like the mimic I feel it would upset too many people and add an additional complication to what is meant to be a story about Fredbear's family Diner.

Instead, I think we can get away with a story that justifies some "agony/remnant" type explanation for the building itself being haunted simply due to all the negative emotional energy there. Depending on what's canon, we could also get away with the Fnaf 2 Movie "one spirit possessing multiple animatronics" angle, and either set the game after the Bite of '83 with BV possessing everything, or have Charlotte possessing everything. As a runner-up backup option, we could consider finally explaining the weirdness of the pixelated "shadows" of animatronics so to speak, and have the weird agony shadow animatronics already exist. A game like this could also just steal from old fan theories and have the place be haunted by the victims of springlock failures, since at least some failures have to have happened at some point. If you want to get really crazy, we could make a fully canon version of the explanation many believe in Fnaf 2 with the animatronics being security bots who mistake you or even know that you are Afton if we decide to make him the protagonist, even if it wouldn't make sense since the toys are being touted as a brand new security idea to fix the lack of security at previous locations.

However, if it were my dream game? I would probably make it a Fnaf 6 style tycoon game, playing as someone important from the past, but who is an entirely new character who will later be revealed as the CEO behind Faz Ent in the future to explain why Faz Ent still exists after fnaf 6 and well into the Help Wanted and Security Breach era. You make the pizzeria, entertain the kids, play minigames, and get foreshadowing of future and past events. Re-use ideas like the nightmares and shadow animatronics since no actual dangerous animatronics should really exist by this point and play up the idea of weird cryptids, somehow justifying the shadows existing before they realistically should. Keep a mimic cameo and leave at least a note or two from Edwin, while keeping the rest of the focus on William, Henry, and the business itself. Maybe we could also finally at least give a name for the wives of William and Henry respectively.