The developers have three years to make their game. Remember, they have three different studios working on three different Call of Duty games. They take turns. Now, I don't work in programming in any sort of way, but I think 3 years for a game like C.O.D is plenty of time. I could be wrong though.
Anyway, on to what I think could give the series a shot in the arm:
1) Change the setting. I am tired of "Modern" warfare. I'm also tired of the near-future setting. Give me laser guns. Instead of frantic beach landings where I watch other boats/choppers get blown apart before I even hit the ground, give me drop ships like Starship Troopers or drop pods/jetpacks like Halo (thank you, Black Ops II, for showing me a glimpse of this). Give me a war set so far in the future (or set in another reality) that the thought of using a machine gun causes the soldiers to either laugh or go, "What the hell is this thing?"
At this point, I am so tired of Modern or near-future that I will even welcome a return to good old World War II. Speaking of WWII...
2) There are other countries--and people from other countries--besides the U.S. Some of Call of Duty 2's most memorable missions have you playing as the Russian or the British. The stronger campaign (to me) in World at War had you playing as the Russians. Hell, Call of Duty 3 had missions where you played as the Polish. The freaking POLISH! A group that barely even gets talked about in WWII outside of being invaded, and that game had missions with them. I get it--this is the U.S. and we are the only ones who can save the world, but damn, we don't do it alone (I'm being sarcastic about saving the world there). If we're going to go back to WWII, how about we completely ignore the U.S. forces, or have them be in the background? There are some great battles in WWII that the U.S. didn't even touch.
And no, having the main character be part of an international force but still be an American DOESN'T COUNT! Looking at you, Advance Warfare! And good Lord, the dribble coming out of Ghosts' characters...is that really how the rest of the world thinks the U.S. thinks? How our soldiers think?
3) Finally, make it COD again. I know that sounds dumb, but the games recently have lost that magic that drew me to them. I want to be part of an ARMY again. I want to storm beaches, defend points, and be involved in a WAR! I'm sick of being part of a one-to-three man team sneaking behind enemy lines and blowing stuff up over and over. I want to feel like I'm in the middle of a battlezone, not in a firefight. I want to hear the AI yelling at each other to get down, only for the sentence to get cut off as one of them gets shot. I want to feel that sensation of hopelessness when I looked around at the wasteland I had to cross to retake the White House, but know that I'm not crossing it alone. I want that feeling of riding in a landing craft as one of the soldiers looks over the edge and yells, "It's the coral! We're stuck on the--!" but then a MG takes his head off. Ghosts' best mission was defending the aircraft carrier because I wasn't alone. The rest of the time it was me and one-to-three other guys who couldn't be killed, but I did all the work.
Black Ops II was the last game to get this right. Yes, there were stealth missions, or small man missions, but there were just as many "YOU'RE IN THE THICK OF IT NOW BOY!" missions, if not more. That changed when Ghosts came out, and no one has been able to get it back since.
So, there are my thoughts: Change the setting drastically, stop making it just about the U.S., and make it feel like I'm in a war again.