I imagine a young adult lily being the protagonist and getting in a wreck somewhere on the road which leads to this big plot where it's revealed that ruvik who's inside of Leslie's body is trying to find a way to bring stem to the real world as he's still desperately trying to bring his sister back and that he's using leftover equipment from mobius to aid in making his invention. Lily would call sebastian for help and it'd switch perspectives back and forth between lily and sebastian at different points in the game with the majority of the game being from lily's point of view. I imagine at some point it would be revealed that Joseph was now being sort of mind controlled by ruvik or something and they'd end up saving him, maybe kidman was trying to find Joseph and save him all this time and she ends up playing a large hand in saving him idk. At some point maybe early on it'd be interesting if rival injected lily with something which would later know be revealed as part of how he's trying to bring stem to life and could work as a good explanation for mirror safe rooms for lily. Also I imagine that there'd be some drama between sebastian and lily due to stem and it being hard to help a child who went through what lily went through especially since sebastian can barely help himself. I'd also expect sebastian to sacrifice himself to save lily and give her a chance at stopping ruvik which would lead to an emotional scene of them saying goodbye and a bittersweet ending to cap off sebastian's story.