Surprise surprise Activision have announced that this year will see another Call of Duty. If you look at what's happened in the past and pay attention to the rumours circulating then it's pretty reasonable to expect Call of Duty Black Ops 2 to be released around November this year. I haven't been a fan of Call of Duty since the 3rd one. Controversially, I found that Modern Warfare was too soon, I would have wanted World at War and Black Ops before it went modern. Since then the series has simply become too repetitive and the competition (mainly the Battlefield series) too strong. Sure the sales consistently break records but how long can that go on for without innovation? Many are simply becoming sick of playing what is essentially the same game over and over.
Though I didn't like the first Modern Warfare I accept that it had a huge impact on the gaming world and, though it didn't create many of the innovations it used, it largely helped to define what we expect from multiplayer these days. After this what has it done? There have been four games since then (World at War, MW2, Black Ops and MW3) and none of them have changed anything significant in the formula. Call of Duty needs to change and I'm sure that if it does it's sales can continue to be high but, if it doesn't, they will begin to drop.
There's hardly a shortage of things that could be done to change it. Vehicles could be added to the multiplayer, the environments could be destructible, the story could focus on one character and therefore be more coherent, emotion could be introduced, as could a sense of morality and the single player as a whole could perhaps be made grittier and more realistic.
Of course Activision could easily be lured into making the mistake of not changing things, it would be easy to make because each game has out sold the last, but it can't continue forever. People will get fed up and will buy different shooters. With the quality of the competition increasing Call of Duty needs to evolve because people cant afford to buy sub-par shooters.