have blizzard make the game :)
guaranteed success.
seriously though
to be a successful game you need a few variable things.
good gameplay
good graphics
good word of mouth (just look at Wii sales- that is almost100% word of mouth advertising)
another way to have a successful game is who the Dev is.
if Blizzard develops the game, instant million+ seller guaranteed
Epic is getting a great reputation as well.
bioware has a good reputation as well
rockstar