It could be a new, super - intelligent animatronic created by combining the technology from both Sister Location and the original FNAF series. This animatronic would have all the worst traits of both, like the stealth of the Sister Location ones and the aggression of the original ones.
Perhaps an evil technician who has been studying both sets of animatronics and decides to use their knowledge to create chaos. He could manipulate the animatronics to turn against each other and the humans in the pizzeria, seeing it all as an experiment. He would be a human antagonist, which is different from the usual animatronic villains in the FNAF universe.
Obviously, Naruto could be the main character. His determination and unique abilities would make him a great fit for dealing with the strange and often dangerous situations in Sister Location.
One idea could be that the characters from Sister Location end up in an alternate universe where they have to team up with the original FNAF characters to fight a new, more powerful animatronic threat. This would bring a lot of interesting character dynamics as they try to figure out each other's motives and abilities.
It could be the malfunctioning animatronics. They are always a threat in the FNAF universe and could be constantly trying to harm Gregory while Vanessa tries to fend them off.
If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.
The story of FNAF Sister Location is complex. It involves a new location with animatronics. There are the main animatronics like Circus Baby and her companions. Essentially, it's about a girl named Elizabeth who gets too close to Circus Baby and is killed. The location is filled with secrets and dark undertones related to the whole Five Nights at Freddy's lore. It also ties in with the previous locations in terms of the company's unethical practices and the haunted nature of the animatronics.
In FNAF Sister Location, the 'baby meal' might not be a typical food item in the real sense. It could be some sort of in - game concept related to the animatronics or the setting. Maybe it's something that is part of the lore, like an offering or a task - related item within the game world.
Obviously, Harry Potter himself could be a main character. He has the experience of dealing with dark forces, so he would be likely to take on the mystery of the FNAF animatronics. Hermione Granger would also be a key character as her knowledge of magic and books could help in understanding the situation. Maybe Freddy Fazbear could also be a sort of main character on the FNAF side, being the leader of the animatronics.
Harry Potter himself could be the main character. He has the experience of dealing with all kinds of dark and strange things, so he would be a natural fit to face the FNAF animatronics in the crossover. He could use his magic skills like the Patronus charm to fight against any dark influence the animatronics might have.
A powerful demon from another dimension could be the main antagonist. This demon could be attracted to the power that Sinbad wields in the Magi world and wants to steal it for himself. He could start causing chaos in both the world Sinbad is from and the world he's been transported to in the crossover.
A new character could be introduced as the main antagonist. For example, a being from another dimension who has been observing both Sonic's and Megaman's worlds. This being realizes that by combining the chaos energy from Sonic's world and the advanced robotics technology from Megaman's world, it can create a new, all - powerful empire. So it starts to manipulate events in both worlds to bring about this combination, while Sonic and Megaman have to stop it.