Sans/reader or even a reverse harem situation where Nightmare (or possibly an evil/corrupted Dream) makes a sort of devil's deal with you
Maybe you feel a bit sad because you're aren't sure if he/they reciprocate your feelings (they do, they're just shy) and you have a low moment
And that's when Nightmare (or Evil Dream) comes out of the shadows to offer you some comfort
(maybe it works better with a manipulative Dream because I imagine a sort of
"I can make your dreams come true" thing)
And because you're feeling especially vulnerable (+plot reasons) you accept his hand and he (either) take you to his dimension where he keeps you happy enough to want to stay and while they don't look exactly like your sans (/skeletons) you're still happy that (what you think is) a version of him is so caring for you
(until at one point that image shatters one way or another)
And Sans (/all the skeletons) realizes you've been taken and goes through a multiverse adventure while you're either being tricked/trying to keep your captor stable and some drama ensues
I'm writing this here because I've had an idea like this for a while but I don't know if I have the energy for it so eh *throws this here*
