A few years ago I
asked Hollywood to stop doing multiple personality stuff as their big reveal.
I still hold the opinion - because I just read the most predictable game plot since it deals with MP - but I've revised my opinion.
Instead of using it as your Reveal, use it as the premise. Start from there. For instance there was a movie whose name I forget that, instead of MP, the villain was stealing people's souls and it just looked like MP. Identity did something interesting with this, but let's even go further:
Let's say that we discover that the main character doesn't just have MP, but the main character is not the primary personality, just one of those created personalities. Furthermore, the main character (and us) discover that the primary personality is awful. They could be a murderer. Or they could be a worthless person. And our main character, who has clearly good traits and such, wishes to "take over".
See? Same typical premise (MP, one personality needing to overcome an otherwise bad one), but we take it to a new place: the one we're rooting for is one of the "created", weaker personalities, not the "real" person. This in part asks the question "So who is the real you, the one you're born with?"