I don't understand, I'm not being too strict about the definition. There's no particular quality that decides whether something is mainstream or not other than it's reach. You could call any game that sells 2-3 million somewhat mainstream, and something that sells 1-2 million would border on it (likely most of it's buyers would be core but not enthusiasts).
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.