Since this isnt a sequel to the original dark PPG movie I doubt she is an unstable creation Professor made years prior who has now come back to fuck his and the girls shit up out of jealousy or anger he had her locked up for the good of the world. So I doubt we will get that juicyness.
I also doubt the 'leak' that keeps getting posted that she is angry at Professor for giving her powers in a freak accident that killed her mom. So in the end I have low hopes for the movie. Best possibility is we find out she is alien, chemical X was her homeplanet's essence Utonium found in her crashed ship, and that the chemical X resulted in the PPG team turning into half aliens who will find out the truth and return to their homeplanet with Bliss and giving the Professor a tearful goodbye thus ending the series or some shit.