I don't currently play with ghosts, but if they were to be improved I could see myself playing with them a lot.
Other people have already listed things I'd like to see, but one thing I haven't seen is adding psychics to the game. This could be a hidden trait that sims can randomly be born with and/or it could be a trait bought with satisfaction points. Perhaps Spellcasters and Vampires might be more likely to be born with this hidden trait and I could see it being hereditary as well.
Those with the psychic trait would be able to see and interact with ghosts, while those without it would only see objects moving about and get a scared moodlet. This would play out depending on which sim you have selected. If you select a psychic sim, ghosts would become visible, while having a non psychic sim selected would leave ghosts invisible. Of course psychics would also learn the Medium skill faster if you have the Paranormal pack, and perhaps living in a haunted house would make ghosts visible to all the inhabitants (but maybe only around a full moon?).
Psychics would also have special interactions with ghosts, helping them calm down, asking them not to break stuff, or, if they're a bit more devious, asking them to haunt another sim. They'd also automatically know how a sim died, and depending on how they improve ghosts, psychics would also have unique interactions in other areas as well. To make it even better, I'd love if Psychic sims would randomly learn something about living sims while conversing, such as traits, occult type, or their specific mood (ie. their top moodlet at that moment).
Well, this is getting a little long so I'll wrap it up, but yeah... psychic sims!