I was thinking, as my poll from 2014 showed that many people wanted Sasarai back, it would be interesting to show Sasarai post Luc's betrayal and death, struggling with his loyalty to Hikusaak now that he knows why he was created.
I was thinking maybe if my hypothetical game were to have two protagonists who meet Viki early on in the story (maybe Viki backstory?) and when she sneezes one of the two is brought with her to Harmonia shortly after S3, where they manage to ally with Sasarai and several other Harmonian characters, possibly Albert, Dios, Lena, Nash (with Sierra), Clive or another howling voice guild member; during this you're trying to escape as Hikusaak treats Viki and the protagonist as invaders, eventually finding a way to harness Viki's power and bring them all back to 471 (temporarily) for the final battle
*Knock Knock* Who is it? Your DOOM! Don't say your doom! Who answers the door for their doom? You're right.
Basket of Puppies!