What if it's a more normal setting? Think about the 'Underground' being a prison. Ebott is an all-monsters city, and humans are outlawed. However, any human that is caught is sent to the 'Underground' and sentenced a death penalty. All humans have tried to escape, and all have failed. Until Frisk comes along.
(Whatever happens from there, and whatever extra details, are up to you.)
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My Undertale Ideas
AléatoireI'm going to warn you now. They are super weird and random, but I guess they're creative. Also, most of these, even if I say I might do them, I most likely won't, so just ask first. And I'll tell you if I'm actually going to do it.