I dunno if I can say much for Undertale post-pacifist because it seems like the post credits scene and the alarm clock dialogue all cover it pretty decently! Toriel's opened a school, Asgore seems content to just stick with his gardening, Alphys and Undyne get together, Sans and Papyrus seem happy to just be travelling around and exploring together. I do like going with the option that Frisk chose to become the ambassador and to stay with Toriel, and I like the idea that Sans got really into astronomy now that he's on the surface. It seems like they all still get together to hang out as a friend group/family for holidays and events in the alarm clock dialogue, so I like to think that that keeps up for years. I know it's easy to imagine things being really rocky for the monsters after being reintroduced to society, but I honestly just like the idea that things go smoother than expected, and while not perfect, everyone is just very content.
For Deltarune, hmm...while I feel like Kris and Noelle would be encouraged and expected to go to college, I'm not sure if Susie would be able to get in...and even if she did, if she would really enjoy an academic setting that much. Not sure if Kris and Noelle would go to the same college, or the same one Asriel is at! I think Susie and Noelle would get together, but it might be a tough few years if Noelle is in college and Susie's not there with her. But I can see Susie travelling around a lot, gaining a lot of hands-on experience doing odd jobs for people, and being seen as dependable in various circles, while stopping by the college to visit Noelle. By the time Noelle is out of college, Susie's amassed all these connections to basically do some real hands-on work with building or landscaping and such, and can support the two of them with that, so Noelle becomes a fantasy writer.
Kris isn't sure what they want to do at first, and they move back home after college for awhile helping around at the various businesses in Hometown, before eventually acting as Asriel's producer when he decides to actually finish and put out one of his video games. Turns out Kris does have a knack for coordinating people and good decision-making, and keeps doing this with various projects for Asriel and their other friends until they're pretty adept at it.
Berdly has pretty middling grades through college but gets really into streaming because he just likes to talk so much, and ends up doing decently for himself with that. Catti and Jockington get together, with Jockington becoming a minor league soccer player and Catti making some cash doing tarot readings. The whole gang see and hang out with each other and their families for holidays and especially for the Hometown festival every year.
