You can zoom in on specific ghost story traditions, like OU ghost stories, Indian ghost stories, etc., or you can do a project where you combine ghost stories from different countries. There are ghost stories in cultures all over the world.
Research Tip: You can get an overview at Wikipedia, and be sure to check out some related terms also like revenant and hungry ghost, etc. There are books about ghosts at the Freebookapalooza: The Ghost World by T. F. Thiselton Dyer and The Book of Dreams and Ghosts. You can also search the entire Freebookapalooza for any book or story with "ghost" in the title: ghosts in the Freebookapalooza.