Sans will be (obviously) seeing Papyrus, as well as his old roommates, Newt and Hermann because they were super integral to how he adjusted to Hadriel and they vanished at the same time as Papyrus and. yeah. he's still very Fucked about that and will probably not be dying but he WILL be killing Shadow's ghost because he's a terrible terrible man.

Jack will be seeing his fake mom and dad! That is to say, he'll be seeing the flashback photograph people who he believes wholeheartedly are his mom and dad but are in fact fake memory implantations that never existed at all because Jack's life is Fucking Tragic!!!! He won't be able to kill them without some help so uh. please help this toddler man. he needs it.