It will be the Stitchwraith?

No, it is imposible. The story of Stitwraith it's the epilogue of Fazbear Frights, and it ends yet. It will be something different.

Before continue

In Security Breach, there are arcade machines that represent one of the Fazbear Fright's characters, Fetch. It means that Fetch exist in the game's universe, what we don't know is if it is canon like a book or like an animatronic.

What will can it be?

It can be a lot of things. First, it must be at the Mega PizzaPlex, so it must be related with the PizzaPlex (it can be a continuation of Stitchwraitch that happends there, but I don't think so). So, like we saw in the Fazbear Frights the epilogue talks about animatronics that don't belongs to the games.
In the PizzaPlex there are zones that are banned and where we can't enter, like the big doors that are at the pirate zone next to the Monty Golf. Maybe there are pirate animatronics there.
Another possiblility is that the animatronic of the epilogue is a unknown animatronic surviving at the Mega PizzaPlex.
Also, the epilogue can be about a human. If it is about a human, it won't be Gregory, of course, nor the girl of the DLC, it have to be about another person.

Finally, I want to know your theories!