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Right now, most MMORPG are "direct combat" oriented. What I mean by that is in a combat situation, there's a group of players against another group of players or NPCs and they hit at each others until one of the two group is down (ok that's the idea of combat but you'll see where I'm going in a few moment).
If we transpose it in the real world, we're talking about the infantry in a real army (ok healers are like medics but that's a detail). But there are many other jobs in the army, from the doctors to the guy who's making the meals in the mess. Let's get back to MMORPGs with an example (I like examples).
Let's say we're in an open world PvP situation in a futuristic environement. On one side, there's a group of players (infantry) ready to attack an oponent base but there's a massive wall around the base and a force field to protect it. I think we all agree that infantry guys can't just shoot at a massive wall, it would be a wast of ammo and that base defenses would crush them. To destroy those walls, we need artillery so players have to operate the batteries, here's a job. But we still have that force field around the base that prevent artillery to hit the wall ( that's future, there's surely a good reason for that), so we need someone to deactivate the force field, let's say an infiltrator, another job.
The whole idea around that example is in a combat situation, there could be many more role to play other than DPS/tank/healer so if a player want to make the job of the infiltrator, he could choose to raise the skills that help him to do his job. Something like stealth to not being detected, explosives to destroy the field generator, hacking to pass through the base defense system.
Of course, not so many players would like to play that kind of role but like in a real army, there's way more infantry guys that anything else but there's always some soldiers to do that kind of tricky jobs. And this only one example, think about things like tanks (not the class, the armored vehicules), air support, doctors, recons and spies ( not just to have the stealth ability like in most games but to actualy gathering informations about the oponent), communication guys (what if the chat system is disabled if the comms are down), officers (to coordinates everything, call air strikes, etc...) and I could go on and on.
I think it could be an interesting approach fo a MMO game, RPG or FPS. What do you guys think?