IIRC, the Mechwarrior RPG addressed the problem of everybody wanting to be a mechwarrior by having players roll multiple characters. So you'd roll mechwarrior, a diplomat, and a crew chief; and let the other players have extra characters to fill other roles as needed.
I think you could adapt that to an MMO so that as you gained rank (levels), it would give you choices as to what kind of alt characters you gain. Those alternate character/roles could gain over time as well. So when you're a lance commander, you have infantry assets; but by the time you're commanding a battalion, you have a squad of killer ninjas!