All of your ideas are brilliant, I'll try to add to them,
Needs ought to be changed, not just scrapped:
-Ghosts should be able to 'Rest in Peace' in their tombstone or urn, which would fill Energy.
-'Bladder' should be changed to 'Ectoplasm' and be fulfilled by sliming sims (New animation).
-Hunger should be fulfilled by stealing a Sims' essence, i.e. Sucking out their emotions, leaving the sim dazed.
-Social and Fun ought to be combined and named 'Haunt', which is satisfied by doing ghostly things, i.e. Haunting and Scaring.
-Hygiene can be scrapped.
Ghosts should also be able to scare sims to death, as a new mean interaction.