I'm thinking maybe a game with two campaigns, one being a mix of the FPS and RPG genres, where you could make your character, choose your squad mates, and all members of the squad at the time could gain experience and unlock new weapons and abilities as they level up, but not so much where skill is a non-factor because it would still be a FPS at heart, and the abilities would be like the perks in COD4 except they would be available in single player and some of them would be things that you couldn't use in multiplayer. I was also thinking of choices that the player could make that would effect the story.
The second campaign be mix of the Stealth and RPG genres, and much like the other campaign it would have choices, unlockable abilities and weapons, but this one would have different abilities and weapons for the most part. To bad it would be demonized like nothing else by Faux news and others.
Story- In about 50 years the US is in a massive civil war over one of many possible reasons , my favorite idea for the cause of the was Religion, say maybe the more extreme religious right made a coup attempt at the federal government, which triggered a civil war between Christian Fundamentalists and well, everyone else. I was thinking that unlike the last Civil war where whole states were on one side or the other, instead I was thinking that the Fundamentalist Forces could have made large gains in the more religious parts of the country, throwing whole states into turmoil in some places.
In this scenario the FPS campaign would be from the perspective of a elite Army Rangers squad which is on the front line fighting FFs(We can use that as a acronym for Fundamentalist Forces) which have managed to take over numerous National Guard Armories.
The stealth scenario would be from the perspective of a FF assassin who's mission is to assassinate the high value government targets to destabilize the Nationalist army and make it much easier for the FFs to make there advances.
EDIT: and this song will make an appearance somewhere,