Normal/Ghost. Saw it mentioned but never elaborated. I think it should be based on the "Halloween costume" style of ghost; clearly someone in a sheet. The immunities would be neat too--weak only to Dark-type moves and Fighting-type moves with Scrappy/Foresight/Odor Sleuth.
For those of you starting topics about the PS4, there's a PS4 board: http://www.gamefaqs.com/boards/691087-playstation-4
Since I can pick a rare but used type combo, I say Water/Ghost.
I've always wanted a Water/Ghost type based on a Siren. The first form could be small, cute but creepy, kind of like male Frillish. The evolved form would be a siren or mermaid, beautiful, mystical, yet scary.
That, or a Water/Ghost jellyfish that actually looks creepy and intimidating, instead of cute and fat. God, Jellicent's design is such wasted potential...