Board Thread:General Discussion v2.0/@comment-27234286-20151118175310/@comment-26277897-20180402171533

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.