Extremely unlikely, if for no other reason that it'd be ridiculously hard to implement properly game mechanics-wise.

How would you feel if you were told that any Forsaken characters you've rolled are getting a forced faction change, yanking you out of whatever guild or group set up you've established up to that point? Add to that the balance issues for PvP, both in having to ensure that the Horde and Alliance race count doesn't get thrown out of whack, as well as this third smaller faction being hopelessly outnumbered in that regard.

I actually wouldn't rule it out, as it has been theorized by people here if the Blood Elves and Forsaken could secede from the Horde and form a truly "evil" faction so that the Horde is no longer regarded as the evil side. But at this point, with Garrosh leading the Horde ever farther from the peaceful teachings of Thrall who had to leave in disgrace, and towards an ideology of conquest by warfare, castes like the Belfs and Forsaken are right up his alley (they wouldn't have had quite so privileged a position had Thrall remained warchief, at least as far as the Forsaken are concerned).