Real ghosts in the game would be nice. We already have these which are potentially harmless, other than exploding upon contact.

My suggestion would be for ghosts in dark areas, such as graveyard, and bits of clockwork tunnels . Kats are considered "ghostly" creatures, but they're cats.

for a ghost to be situated, I can only presume it would be best for these new "ghost" enemies to teleport. Imagine this:

You are walking. You see an animation of a ghost being summoned. It teleports to you (or your party ally[s]) and attacks. The special thing about ghosts is that they can teleport quickly. They can often teleport if they feel they are "over-attacked". Sometimes these ghosts can teleport away and duplicate themselves.

Refer to this video for closer reference.