Third faction as the Forsaken.
Races are : current races as forsaken (undead human, undead orcs, undead taurens, etc.)
You became a forsaken a level XX if you choose to.
Victory to the Alliance !
Now look at Star Craft 2, between Terrans and Zerg we have more lore, introduced to the movers and shakers among the NPCs and created over arching villains out to destroy all life (Amon and the other Old God like bad guys).
Also if note in the last Zerg game was the focus upon the Zerg moving from a hive mind to being free willed, the hive mind was a corruption of the proto-zerg to help the old god like beings destroy all life in the universe. The hive queens were being genetically upgraded to think for themselves and to have vision to make them more deadly (not vision as in sight).
This is all paving the way for a 3 faction Starcraft MMO where players can play zerg characters and not be mindless hivemind servants.
Creating an MMO wont hurt the esport, the esport will still continue because it has its own niche that is popular and keeps the bucks rolling in, but that does not exclude the creation of an MMO based on the same intellectual property.
No it wouldn't make sense gameplay wise and is an overall stupid idea. They will never do that. That would basically give you free characters of every class that are already ahead in levels. They won't let you to skip leveling that many characters. It would also cause imbalance because when that whole faction can hit max level so easily, everyone would roll it.
We have faced trials and danger, threats to our world and our way of life. And yet, we persevere. We are the Horde. We will not let anything break our spirits!"