Toriel became a massive hoarder while being an extreme hermit now. Don't expect her to take care of you, she's a wreck.

Sans lost a barfight and is currently healing, he's weaker and has a nice shiny eyelid (don't ask how that works)

Papyrus got a new Bone Canon from Alphys, and some more gadgets for his puzzles.

I have many more ideas for this but I'm just writing down what I have made for these 3.