series that have huge sales are usually pioneers in their style of play....or radically changed (GTA3 became 3D) to get there.
So really there is no way to forecast this as it is gameplay decisions that makes the shift.
One thing is certain though... some people know how to do these games... Nintendo, Blizzard to name the most successful ones.
Music games were popular due to novelty gameplay.... it faded.
MW is a niche, there are many better shooters but this one is "the one to play"... it will eventually fade with no replacement.
GTA has also exhausted it's novelty...but more so due to competition/diversification in this style.
Gran Turismo was one such franchise in a well furnished area (so all the more impressive)... but they just didn't manage to hold on to their audience...
Next mega seller franchise will either be a new Nintendo franchise... or a new game from a developer that does not sell millions currently. We're definitely not going to see ME3, FableIV, Uncharted3, street fighter, Assasin's creed or any other copy/paste game start selling 15M anytime soon. Heck even Battlefield 3, a solid PC franchise, is going to struggle to get to 15M on 3 platforms.
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