The red headed step child of FPS these days is the campaign. No one cares about them, no one bothers playing them or acknowledging they exist.
Not surprising. The cash cow of FPS is the online experience and a developer would be a fool to ignore that or even invest time anything other than the online aspect. But there's so out there that wish campaigns were taken more seriously. There's still a lot that can be done.
Procedurally generated content is about to explode onto the scene. Several games including Persona 4, Destiny, and a whole plethora of indie games are using this as a tool for enhanced replay-ability. The biggest drawback to the campaign is that once you beaten it what else is there to do? Collect all the intel or medals? Ain't nobody got time for that. Procedurally generated maps, enemies, missions, and events can change all this. Loading up the game each time could be a new experience and that's what players look for when gaming.
Multiplayer is also a huge people pleaser but not just with competition. Modes like Zombies and Extinction are hugely popular. Clans also give evidence to the desire to Co-Op with friends. Adding online Co-Op to a procedurally generated campaign with events and a small mix of PvP could become a new staple for the series.