One of my secret weapons of productivity is to always keep a full pot of ideas on low simmer in the background. When one projects ends, I can take a short break, look in the pot, see which idea has floated most to the top, then scoop that off and set to work on it. At any given time, two or three ideas remain near the top; the rest sit at the bottom and wait. Some of them are no more than "I'd like to do something like this" ideas; here's a few.
A story set either in the cradle of civilization (ancient Mesopotamia / Egypt) or some fantastic analog of it.
Ditto above but an amalgam of China/Korea/Japan. I got very close to writing this a while back under the name The Palace Of The Red Desert, but I had to scrap it when I realized it wasn't nearly ready for prime time.
A story set impossibly far into the future, involving a cadre of alien species, none of whom are human, and who must overcome all of their apparently irreconcilable barriers to solve a greater mystery.
Something about video games, virtual spaces, etc. Some of the ideas originally doped around for this ended up in Welcome To The Fold.
A thriller about two siblings who are at odds about what to do with the man who ruined their lives. (More than that would be telling ... you know how I am about spoilers.)
A woman who may or may not be an alien on earth is obliged to take responsibility for her sister's daughter, which potentially jeopardizes her larger mission.
And finally: Shunga-Satori, which I'm tentatively calling my "sex and transcendence" story. I couldn't even begin to describe this one yet. I fear I never will.