I have some locations and characters in place. Gozer was going to be mentioned. I like the Lovecraftian idea. That would make a nice excuse for multiple boss battles. Dana will be a part of the plot, but not the target of the ghosts. She is pulled into the story almost on accident.
i would go for set as a potential new villian because he is the egyptian god of chaos: and chaos is unpredictable… so i would bet if he was a villian he would be too.
his powers would be able to do anything that would be chaotic… (not the show).
maybye have a rivalry with gozer perhaps.
hard to really choose. there are so many good monsters, ghosts and spirits in fiction to choose from.