I think if SC ever becomes an MMO [I've thought about this for hours, believe me.]
Zerg will be 'grinding' npcs.
Toss will be hard for noobs, set up for your 2nd character.
Terran will be the starting point, that isn't to say they'll suck, toss and terran will have seperate styled gameplay. Toss, working in one of the castes, as khalai, templar or [the other one i forget] terrans choose from firebat, marine, or ghost. As they go up levels, each of the infantry types gets access to vehicles and better weaponry, and eventually chooses an allegiance. [korhal, dominion etc.]
So lategame, you could have a level 70 rine, siege tank or wraith, which you fly around pwning little zergies with.
PvP works like SC as it is, once you reach the level cap [100?] you can elect to become a commander, commanders must pay a BIG fee, and be approved and watched over by administrators [there's only a few commanders per server, 10-20 online at a time] and then you go play melee matches. As a soldier, you go to a window, and decide 'Join battle as [Unit]' unit being whatever you are, you go into a queue, and as soon as a commander builds your unit, you enter the field ready to kick ass. You get orders and fight to the death, gaining XP for kills. Battles go against Toss [other players] Terrans [as toss] and Zerg [easy beginner pvp, mostly tutorials, less exp but still good later on, these games are controlled by commanders too!].
I haven't thought about toss much, but whatever they do, I want MMOFPRPG
None.