I don't know how quickly the setting of "modern warfare" can wear out, as it's been banged at for well over half a decade now and there have been many more games with that setting compared to those with the WW2 setting. Myself, I don't think I can get tired of it, since I play these games primarily for the modern weapons that I love.
Don't try. IW has never released any games in subsequent years, and Activision's franchise structure between software houses prevents that. Many months ago, an employee leaked that Treyarch has got this year's COD entry covered, and simply referred to it as "Call of Duty 7". They probably didn't have a real title at the time. After that, I'm guessing Sledgehammer or whoever is handling MW3 will put it out, although I wouldn't mind another year of delay, seeing a 2012 release.