Zhukov said:
It's already been done. Planetside 2.
Three huge maps, each divided into territories that are fought over by three somewhat distinct completely player-driven factions. Infantry, AFVs, tank and aircraft. Think Battlefield, make it 10x as big, but without the fancy visuals and sound.
Pretty much the only MMO to ever get and held my attention.
Mate, if you think Planetside 2 was awesome, please tell me you played Planetside 1 at least once.
It's probably barren now that 2 has come out... but damn, Planetside 1 is easily one of my favourite shooters and MMOs of all time. It may not have the greatest "shooting" or graphics, but the gameplay was literally the most fun anyone can have.
The one thing I didn't like about Planetside 2 (well, this and the download size which has kind of turned me off playing more) is how it seems a lot more, well, "Match-y" than 1. You loaded in and joined a "match" rather than working with your faction to form a supply line of vehicles and transports. Which leaves chances for the enemy to ambush this line. Which gives the Outriders on bikes and such a job. See, in 1 you had people running scouts and such, you had the factions forming war parties (seriously, if you haven't seen a field nearby a base swarming with cargo-ships dropping vehicles off, with MECs setting up, soldiers running in between and then the order to go... Damn.)
It really shows that the greatest fun out of that game is to be a tiny part of the faction. When I buzz someone in my Mossie or Reaver and see an infantry squad take out the Anti-Air mec troubling me as our tanks advance and destroy their gate defence whilst my wing-men form up and take out the enemy scrambling to get in the air... Ah, fun.
OT:
So yeah, Planetside. 1 and 2. And shit. Now you've made me want to download Planetside 2. Damnit Zhukov. I'm a bit iffy though, not sure if it still has the playerbase.