Sans could play an unfortunate part in those "one of their friends is trying to kill them" dreams. Especially since, in his dreams, he could still expect "Chara" to look like Frisk. So Frisk could find themselves in the last hallway and things could go horribly wrong from there.

On the other hand, winding up in someone else's memory of Papyrus could do him a lot of good.

And I actually had a dream idea for Sans-Serif that would involve a lot of crowds, so maybe that could take place in their home city?