I had a couple game ideas as a kid that have, shall we say, been "taken" over. One was "SimFish", where you'd, like, build your own reef and fill it with all kinds of undersea life--I saw some DS game like "My Aquarium" or something like that at Game Crazy the other day that was basically that (though I knew the concept had already been done).
One idea I came up with, though, was going to basically be a 2-D platformer/racing game. Another was a more complex, full-length game built around the concept of those "Purple Pipe Puzzles" (as I like to call them) in Wario Land 4.