I had given thought to an MMO version of ARMA and concluded thus. It could work. Here's how I would see it being successful.
1) Sandbox style expansive maps --> Player created cities based on Private Military Groups(Clans/Guilds/Etc.) that belong to a nation(Race/Faction/Etc.). Each nation would have two main cities/"safe zones," that the players could trade in, sell items, buy items, etc. Furthermore, players can invade other countries to gain land and loyalty points with their country to use to buy country specific items. Boundaries should probably be reset every month give or take so one country doesn't take over the world and end the pvp.
2) Expansive Crafting System --> Player created weapons and ammunition, vehicles, houses, bases, fortifications, etc. This would create a player run economy supplemented by national origin. I.E. crafter from U.S. gets enough loyalty to get blueprints for a certain...helicopter, house, fortification, gun, etc. that only U.S. players can make. And on the other side Takistan players can make certain things, Russian players etc etc. Furthermore, there should be different qualities of materials that randomly spawn throughout the world for players to "harvest" and fight over for their countries. These spawns should probably reset every month give or take so that not one country can horde the materials and also create conflict over said items to spur pvp.
3) Insurgency --> Pretty simple, randomly spawns players every minute(as a group of players planning on using this) in a random country to start terrorizing the residents of said country. These would have no nationality and would spawn with basic items(AK47's and a few grenades. Probably even with an off-road jeep) and could rank up within the insurgency tree to get better gear at spawn.
4) Skill Trees --> (For all of those who played SWG precu you will understand this) No levels per say(perhaps ranks, but those won't get you anything other than say a better looking dogtag.), instead you will have different skill trees to work on. You won't be limited to one skill tree, but different base trees will lead on to more advanced trees and combinations of base trees will lead on to even more advanced skill trees. The way you balance this is by making each option in the tree cost a skill point and give a total number of skill points to each character. For better understanding I will just refer to .
Personally I think the idea of an Arma MMO would be interesting no matter how they did it.
