They really couldn't because the Legion is the "big bad" of the universe. Having them be a third main faction would basically lead to a leveling system with virtually no NPCs if it took place on Azeroth.
These, it would be more plausible to have the forsaken create a 3rd faction with those that were in the past part of the Scourge, etc...
Why? cause that faction won't be like the bad guys (well at least not always), and they have a big option of races that could be part of their faction
Originally Posted by JTHMRulez1
The Godwin Law of WoW is obviously the "Sylvanas is evil?" Discussion.