>>30692133>Post-game>Portal opens>Something went wrong with Bill's separation attempt with the teleporter>Lusamine, Lillie, and Bill end up in the MD world>Lusamine is Nihilego>Lillie is Cosmog>Bill is Clefairy>Do missions to help Bill gather the materials to get them home>After a sad argument with your partner about actually being able to return to your world, (You) decide to stay as the way to the normal world is a one-way tripThat's the most likely way I could see it playing out.
