The Name Game
First of all, I don't imagine it would be 'World of Starcraft'. It's a sweeping saga set against the backdrop of limitless space! Maybe 'Galaxy of Starcraft' or 'Universe of Starcraft' is closer, but they don't have quite the right ring to them. And even if it were a single world, which world? Char? Aiur? Korhal? Here's where we can begin to make the WoW comparisons in earnest.
WoW contains 2 factions, with 5 races each. Conversely, Starcraft has 3 factions, and each one is its own race. Each race has its own starting area, which in general terms is its homeland (caveats about the Draenei aside). So, we might assume that if you get the option of picking your character among Starcraft's 3 races, their starting areas will be their homeworlds. For visibility's sake, let's let the Zerg have Char, the Protoss get Aiur, and the Terrans start on Korhal.
In a class of its own
There are 9 character classes in WoW, and this is where it gets a bit tricky. Depending on how you look at the hierarchical structure of units in Starcraft, the total number of classes is a bit harder to calculate. Each race has its own specialized units, and they're not shared between factions, the way many classes are in WoW. For instance, the Terrans have Marines, Firebats, Vultures, Goliaths, and many others. It doesn't seem feasible to consider these playable classes, especially when taking into account the entirely mechanical nature of some of these units. The Zerg and Protoss have similarly unique units to themselves.
So, when you create your character, what choices would you have? Continuing along the Terran line, let's say you start off as a basic Private. You could choose to specialize in either Infantry, which allows access to the Marine, Firebat, Medic, and Ghost classes; the Ground Vehicle set, which includes the Vulture, Goliath, and Siege Tank classes; or the Air Vehicle set, which gives you the Banshee, Wraith, Dropship, Valkyrie, Battlecruiser, and Science Vessel. I'm deliberately leaving out the SCV, as WoW doesn't let you play as any of the resource-gathering units from the Warcraft series.
Looking at this wide variety of classes to play, it seems obvious that a great way to make this work is to allow players to pursue one path at a time. For instance, let's say you wanted to fly. You could start off learning the Banshee, getting upgrades during your tour of duty, then perhaps once you'd earned enough flight time, move on to the Valkyrie, then to the Wraith, and finally to the Battlecruiser. It would be a simple progression from least to most powerful. The same system would allow you to pursue the other two paths, and you could probably choose to switch between them at any time, with a corresponding point penalty for switching horses mid-stream.
The craziest org chart in history
Further, the Terrans, being a military organization, have their own hierarchy of rank. On a purely leveling basis, your rank could be tied to your combat experience, with your class level being a separate metric. Higher ranks confer greater HP, perhaps access to more sensitive, 'need to know' areas, and the ability to command more units beneath your rank, though that might only be practical in PvP arenas.
But again, would this structure work with the other 2 races? The Protoss have a somewhat similar organization, but the Zerg are a hive mind, each unit controlled by its Cerebrate. Compare this to the loosely-structured WoW, where rank is merely enforced through greater levels, and we begin to see some issues arise. It's conceivable that rank among units might be dismissed entirely for the sake of gameplay. Let's not worry about that right now.
However, unit balance is something that does need to be considered. In the original Starcraft, you frequently saw a single Protoss Zealot take fire from numerous Marines without immediately falling. Obviously, this won't work in an MMO setting. Thinking of one-to-one interaction, though, would our hypothetical MMO have leveling combat like WoW, which means random enemies wandering around an area just waiting to be taken down? That's always an option, but within the context of the overarching story, would it make sense for a Terran Private to be let loose in the wild just to do some hunting? I'd hate to see this, but it's possible for the Terrans to create a 'holodeck' for training purposes. This could allow for worry-free leveling up, but as an artificial method, it instantly drops suspension of disbelief.
Such conceits make more sense with the Protoss and Zerg. The Zerg are a ravenous race, with 'Survival of the Fittest' being their unspoken motto. It's not much of a stretch to assume they'd wander around fighting and devouring anything they could. Similarly, the Protoss could require its warriors to go on rites of passage, a 'blooding' being necessary for inclusion in the tribe. But the whole reason you sign up for a Starcraft MMO is to battle the other two races. So how do we let a new user jump right into the action?
Player vs Player vs Player
Perhaps it's here that Blizzard would differentiate its two flagship titles. The Starcraft MMO could be all about PvP within a larger group, no soloing possible. This is, arguably, what MMOs are all about, after all -- the shared experience. And by this time, WoW players have learned how to create teams, have learned which strategies work and which don't, and how best to organize raids. This would merely be the next level. Would Blizzard be crazy enough to create a PvP-only MMO? Sure! They have carte blanche to do whatever they want, and the resources to fail. But given their 'It's done when it's done' attitude, and their continuing track record of successes, it's probable that they'd find a way to make even this sort of scheme work. And if it did, soon others would copy the gameplay, and boom, a new MMO genre is on hand.
So, does this make a Starcraft MMO seem more, or less likely? There are a bunch of concepts inherent in Starcraft's gameplay that need to be translated to MMO format, or it's not a Starcraft MMO at all. What's the overarching story? Containment of the Zerg in their mindless desire to consume all life? Preventing the Terrans from creating something they can't control using their mad scientific experiments? The Protoss' search for a viable homeworld? What about the hero units, Jim Raynor, Zeratul, and Kerrigan? At a base level, would we just be replacing magic with psi? Time will tell, of course, but if nothing else, these are ideas that will probably appear in some form or other in different places, as the MMO genre matures, tries new tactics, fails, and moves on. Whether it be Starcraft, Diablo, or an entirely new IP, count on Blizzard to be at the forefront of the next wave of innovation.