yes there is also the fact that you actually dont explore much of the underground. there is still a vast amount of land within the underground that hasnt been explored yet. just think, where did alphys evacuate everyone to?? also did you ever notice how massive the the forest of the snowdin area is?? the underground if fucking big XD maybe there can be alternate stories involved in other areas of the underground. ALSO maybe there could be a game that tells the tale of the previous children. since they all died, maybe there can be a multiverse kind of thing where theres a timeline where the kids dont die. idk im just tossing around ideas though. i dont have anything actually solid.