It seems almost impossible to come up with an idea along these lines without being REALLY redundant. For example: a lot of people are hoping/anticipating/wishing for a "Metropolis part 3," either as another album or a single track.
One idea that I had was an album that has more of a common "theme" or "thread" along the lines of Alan Parsons, Pink Floyd, or "Six Degrees" Maybe a collection of songs about different religions, or wars or how various countries were founded. Or different inventions, such as those of Benjamin Franklin. If anyone could make those often boring subjects interesting, or even exciting...
I know a lot of people don't like the whole Sci-fi/horror/dark fantasy thing, but for those of us who do: A collection of songs based on short stories by writers such as H.P. Lovecraft, Stephen King, Clive Barker or Dean Koontz. Or each song could be based on a different story by a different writer, with something linking them together.
Speaking of Mr. King-and I know this has been done before in songs by bands such as The Alarm and Anthrax, but-I wouldn't mind an entire album based on "The Stand."
Or another original story about the conflict between good and evil, along the lines of "In the Presence of Enemies."