"Leaps and Bounds"? Seriously? I hope that's sarcasm.
Anywho if they want to remain an Arcadey Game Series all they have to do is change the weapns, perks, killstreaks, maps and add more story and make their fans want their game with youtube trailers.
Only way they can really improve is if they cross over into another sub genre. So for instance, make a sort of Open World FPS or something easier and simple than that and go with a more realistic approach and add entirely new mechanics (while still retaining the old formula and engine) and dynamics.
If they don't care about making a game that requires more skill than all they have to do is go in the direction they are now with being able to have a Claymore, Portable Radar and heartbeat sensor all on one class.
It really all depends on which direction and/or sub-genre they want to take COD in.
If IW and Treyarch intend to carry on with COD the way it is and feels now well then with the next generation consoles they better hope their Community is more sycophantic than they are because I'm pretty sure there will be games that will outsell and outdo COD when the next generation consoles arrive.