I found this Roleplay that is meant to tell the story of everyone after the events in Undertale's Pacifist Ending. I've enjoyed it and decided to help promote it. So here's the link and a bit below to talk about it to help entice you to try it.

SPARED AN UNDERTALE RP


So, Frisk saved the monsters of the Underground. They are now free to see the surface and enjoy the future. But is it all truly over? What becomes of our favorite characters or even the other monsters? What bout the humans? Some may come to give the monsters a chance, others, might oppose the idea and campaign towards getting rid of the monsters again. And what of Flowey? Better yet, what of Chara? Is it all really over, or merely the beginning?

This roleplay focuses on the lives our favorite characters (and Jerry) will have on the surface. Hanging out with each other, meeting new humans and monsters, and probably even facing new problems that arise with a world that may or may not want to coexist with Monsters. There might be some drama, bloodshed, and action! The possibilities are endless! (Including more chances to ditch Jerry)

So if you're looking for a noncannon continuation of the Undertale story and want to fill the role of either your favorite characters or make some new guys up. This roleplay is a good opportunity to do so. Just click that link above if you are interested.