The yearly installments of COD differ more than the yearly installments of many, if not all yearly sports games. There's a new campaign, new weapons and new online modes. There's also tweaks to the engine, so it looks and plays better. These changes could not be added simply by a patch, and it would not make sense from a business stand point to do so.
The FPS genre is more flexible I would imagine in terms of creativity than say the music playing genre. While the music genre pumped out yearly installments of games that had little to no difference between them save the music list, the FPS genre is always experimenting, adding and looking for something new. It's also the most popular genre in gaming at the moment, and has some of the most talented studios developing in the genre.
Future improvements this generation would simply be more features. Add co-op, or online co-op, or 4-player split screen, or exciting new modes or even dinosaurs. Anything new really.
