More of a recent one, but Bulletstorm 2. Apparently it was in development for a while until Epic took People Can Fly off doing that and onto....Gears of War:Judgement....oh....
I absolutely loved Bulletstorm, so it's a bit of a personal gripe for me, but annoying to know that it could have been, but now probably never will, seeing the first game didn't sell as well as expected. Although it has been said that it may be created in the future, but from personal experience I know that it is just a kind way of saying "Nope, never gonna happen, but we'll say maybe to keep your hopes up". On top of that the game finished on a cliffhanger, a fairly interesting one at that, which makes it even worse. But I could be wrong...god I hope I'm wrong.