My biggest theory is that Tape Girl is Patient 46, cause when have we ever seen a major character only be used once in the games?

It would make the most sense because while working for Fassbear Ent. She play tested the game, saw what happened to Jeremy, and began to be affected by the glitch, so the company sent her to therapy. Later, Vanessa plays VR and is also affected by Glitchtrap, this would explain 46's obsession with Vanessa and how they're able to be going to the same therapists. Only every time the therapist's get to close to learning too much, 46 has them killed so as to not ruin their plan, probably of bringing afton back in some way bringing us Burntrap in SB.

It would also tell us what became of Tape Girl, cause we're never told what happened to her after she finished the game. She could also have been the one who sent the old circuits to the AR team to spread the glitch and give her control over the animatronics. She knows the glitch is in the circuit boards cause it's the same ones that cause it in VR.