Board Thread:General Discussion v2.0/@comment-27234286-20151118175310/@comment-34645353-20180402232029

Freddy Fanatic wrote: A prequel could be detailing life before the monsters were sealed in the Underground, revealing more about Gaster’s role, or detailing Asriel’s life with Chara prior to their deaths. A sequel could be a new threat to monster society on the Surface, threatening to orchestrate another war, or maybe something much more sinister. A spin-off could be detailing another location that exists within the world of Undertale, utilizing previous lore, while introducing new elements within the Undertale universe. Or perhaps focusing another monster’s perspective.

There’s a lot you can do with Undertale’s story, and branch out as well. Again, a sequel could only work if you did True Pacifist. Since there is no "true ending" to Undertale, it's impossible for a sequel like that to exist.

A prequel would probably be the most logical idea if you want to clear up some mysteries. However, that could go a lot of ways. It could branch out Suzy's story, Gaster's, the skelebros', any monster in the game really! Maybe the prequel could have you play as different monsters and tell different stories? I would really love to see that.

Now, a spin-off would be the most unique choice in my opinion. I don't think a spin-off could clear much information, but it IS possible. I have a few other ideas for a spin-off:

Playing as one (or more) of the fallen kids. This might be able to clear up some mysteries, too. Maybe one of the kids met Gaster, or was helped by Flowey, or something. It would be really sad to see your character die at the end, though. Playing as Chara would be really cool, too!

This is a stretch, but... maybe as a monster in the Underground? Or multiple? Heck, maybe even play as Flowey! Any monster would work in my opinion. Even one like Froggit. You could see Frisk go through all of the routes. If Frisk went through all of the routes, though, it would make the most sense to play as Flowey in my opinion. This wouldn't be able to clear much information in most scenarios, but it may be possible to.

I also like the idea of discovering a completely new location in the Underground that you said.