Here's something I posted on a different website's forums a while back. I have some tweaking to do with it, but the basic ideas remain the same:
Setting: North America (mostly U.S.)
Time: Near future (2010's)
Story (FPS): It kinda goes off of the "Red Dawn" sorta feel. You are a U.S. citizen, going about your everyday life. One day, a foreign force invades the U.S. via the Mexican route to split the country in two. As a civilian under attack, you and a platoon-sized group of men form a guerilla/militia group to disrupt communications and supply lines in order that the regular military forces can defeat the enemy on the fronts.
As the campaign progresses, you develop relationships within the group, enough to the point of being emotionally pained when members are KIA. You also develop certain skills: over the campaign, you develop better aim, more stamina (faster run or more carrying weight), and the ability to perform new tasks: explosives, medical assistance, fixing weapons.
You would also acquire "better" weapons as the time progresses. At the start, the player is equipped with basic, civilian-available weapons: pistols, semi-auto rifles, hunting rifles, basic dynamite for explosives. As the time goes on, you acquire surplus munitions and weapons from regular army stores, as well as the enemy weapons: limited and basic automatic weapons ("old" M16A2's, M-60's, M249's), claymores, grenades, and C4 as explosives, and basic anti-tank weapons: LAW, SMAW, Old TOW and Dragon missles, not so many fancy attachments and toys though. And, once your "platoon" begins to be in contact with friendly regular forces, you get the regular army weapons and equipment. The ultimate goal in the game is to help drive the enemy forces out of North American territory. That's the basic gist of it.
Story (MMOFPS): This is the multiplayer installment of the game, much like how CS is the multiplayer component to the half-life series. Your character functions more like a mercenary: you are solitary, until you join a clan or group up with other mercâ€™s. You roam through this world: probably an island about the size of a small state, and perform tasks for a side of an ongoing conflict between two nations. For instance, a mission for Army A requires you to go into an Army B outpost and destroy a communications tower. Problem is, other mercenaries are being paid by Army B to defend that communications tower. If you succeed, you get money. If you die, you spawn at an origin point: a town or city on the island somewhere, and have to find a new mission to do.
The weapons system in this game is sort of different too. As a mercenary, you start out with vanilla weapons: plain, â€œstandard issueâ€ assault rifles with no attachments (M16A4, AK-107/108, SIG 552, G36, Galil MAR, AUG-A3, F2000). But, as you accumulate money, you are able to buy more weapons: pistols and SMGâ€™s for secondary weapons, more sophisticated rifles and carbines, light machine guns, shotguns, sniping rifles, and attachments for your weapons. You would also be able to buy better gear such as body armor, magazine pouches, and camouflage for sniping work. All of these having real pricing, and that money comes from doing missions for either army.
Generally, realism is the key to success in the game in all aspects: weapons, movement, injury model, and sound.