A few days ago I came across news about a game named Man vs Machine which planned on hosting a game for 1,000 players simultaneously. It would appear they have done it. While they never peaked at the 1,000 players they had hoped for they got extremely close falling only one player short of their target. Setting a new world record for most people on a single FPS battlefield. I completely forgot about it on Sunday night. I could have been that 1,000th player.
Anyway it is a hell of an achievement and it would be cool if in the future we saw full blown FPS MMO. You could take make a war shooter where you have a number of squads in various locations. You could have a bunch of people playing as pilots flying to transport players behind enemy lines. The people dropping in to enemy territory could have to hold out until reinforcements come along with the time on them getting to you depending on how well the other team is doing. The possibilities for a shooter with say the numbers of an MMO you could completely re enact wars by making it co-op rather than pvp. Something like that is probably still a fair way off, but give it a few years and I can see them popping up.