A free-for-all Planetside... Let me explain.
Take the basic FPSMMO. PCs do not level up, but equipment is complex, highly moddable, and both limited and droppable. The terrain is taken from Outerra- that is to say, an earth-scale planet with procedural details.
Players team up with friends to from gangs or corporations; your team can take procedural-generated contracts to collect goods, control sites, take locations, and so on. Contracts go out to a selected assortment of other groups, and they are always paired in a matchmaking way- so if there's five of your friends online, your team and another team of five might go up against a large team of eight in a head-to-head fight.
Above the small scale, corps can form alliances and contracts in a tiered structure. You four guys are Delta Team, and you work for the Black Division, who are a part of Checkmate, Inc., who work for the Applied Tactics Alliance. When Applied Tactics Alliance gets a contract like "Take out Whitewater Refinery," all 98 players, including you 4 from Delta Team, are invited to participate in an attack against the 45 defenders there.
These contracts are on a time-delay, anything from 5 minutes for a squad-vrs-squad deathmatch to a couple days leading up to a cross-continent mega mash battle (probably involving many sub-objectives orchestrated by the leadership of both sides). You can set your calendar to be online for a big battle, or participate in a smaller battle of a major war.
Furthermore, players and groups have bases and facilities under their control that provide bonuses. Players can make contracts themselves to either recruit allies or to give orders to their own organizations, or even just place bounties on other players.
Between site bonuses, cybernetics, weapon upgrades, and customized vehicles, an advanced player can be immensely overwhelming to a new player; however, the best equipment is "unique," meaning that you can permanently lose it in combat, and others can steal it. This sort of thing happens when you lose a "major contract" or when you lose your main base, although if you don't perform risky missions there's a chance you'll lose it anyway (to prevent the best items getting troll trapped). The result is that the most powerful players are also the biggest targets.
But procedural contracts and pitched battles are only part of the game. The big thing is betrayals and civil wars, because those unique items and facilities have very limited output, and it's always better to split the prize 4 ways instead of 5. And 3 ways, instead of 4....