It can be any game, including ones that have already come out.
My choice would be COD 4. If that game didn't come out, COD wouldn't be all the rage and the console market would have way more creative games.
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Metroid Prime. I didnt really like it, and according to World Records, Metroid Prime is the most critically aclaimed Sci-Fi game. Halo is second. So if Metroid Prime never released, Halo would be the best Sci-Fi Shooter.