Assuming that things happen akin to how UT Paci-Neutral ends (Asgore dying, Flowey taking the SOULs and fighting you as an ms paint monstrosity (yes i know it’s Photoshop, don’t @ me.) ) the human would defeat Photoshop Boogie, decide to kill or spare them, but then unlike how the original Paci-Neutral ending went, the Human would be unable to leave as (this is going off the theory that UT Frisk used Asgore’s Soul fragments or whatever to cross the Barrier) Sans’ Soul would’ve shattered immediately as he aint a Boss Monster (if I’m wrong about this and he is in SS, you can lemme know). Thus SS Frisk would stay underground, help monsters (and Chara) through the likely new changes (This assuming Papyrus takes the throne), such as setting puzzles and japes to test new humans, and other such Papyrusy things, but also helping find a new means through the Barrier. It’d basically be like the Paci-Neutral phone call but with SS characters and circumstances and Frisk actually there. Then eventually Boogie turns up, spouts stuff about a “better ending” just like Flowey does at the end of Paci-Neutral, and SS Frisk resets. As for what they do next, who knows? All of this is just a theory anyway. Could be that there is no paci-neutral route and just a pure Pacifist one.