If that old rumor about Grabbed by the Ghoulies originally starting out as a Harry Potter game has any truth to it whatsoever, then that one. The idea of Rare making a Harry Potter game is really intriguing and I'm sad it never happened. I'm guessing this was probably when they were still with Nintendo and this would've most likely been a GameCube title, as we know that GbtG started out on the GameCube.
Not to mention all the other games we could've had from them on the GC if it weren't for the buyout. Hell, even under Microsoft, they had tons of projects that never came out.