Guide them up from the underdark to the surface to see the sun one last time.Can't remember if they've left the oven on or not.Dead chef ghost, cannot rest until he proves his signature dish is better than his still living rivals dish.Have a treasured possession buried with their corpse.Return a stolen good to its original owner.Tell their kid not to follow in their footsteps.Find what was at that damn spot marked out on the old map they found.Preserve their tattoos, possibly through some very gross taxidermy (They tell a very important story).Possess a commoner for one last bout of carousing (maybe a PC?).They were listening to a beautiful song when they died, they need to hear how it ends.

Find that stupid treasure they have been venturing for all of their life.

Make sure their lineage remains pure and the children are responsible with their inheritance.

Tell the person who killed them that they are forgiven.

Find that one last item that would have completed their (very esoteric) collection.

Finish climbing that damn mountain that killed them.

See land again after being trapped for centuries on a ghost ship.

See the ocean one last time after dying in the middle of the desert.

Realize they are actually dead (à la Sixth Sense).

Figure out where they heard that music that was going through their head when they died.

See someone/something they were waiting for.

Fix the error in the formula they were writing.

See a child/mentee get married/graduate.

D and d ghost