There are so much they could do with improved ghosts that it's almost funny. Specific kinds of haunting for your ghost, like the water tap. The ghost could simply destroy it (small loss of ectoplasm) or it could absorb water and then for an hour or so, wherever the ghost goes, it would leave puddles of water in its wake, to other Sims joy. I don't even have to say what it would do with a fireplace.
And then ghost aspirations.. this would be something to implement. But would it be brand new goals or would the Ghost's aspiration in life be locked in, and once you complete it, the ghost will find peace?
I can't even begin to cover everything a controlled/uncontrolled ghost could do. But I like the idea of an ectoplasm meter depleting slower and slower as the ghost grows in strength. And I guess if you wanted zombies, a dark way would be for the ghost to visit a departed Sims grave. Too much for Sims?
Again, the possibilites would be plenty. It would make for one heck of a gamepack.