Well, this is just more pure fan speculation here, but...
If the meta-crisis Doctor is NOT the Valeyard, then how could such a being be generated? Perhaps it will have something to do with his next regeneration...maybe the regen process will reverse the effects of Clara's mindwipe, bringing back all the memories and all the negative things he did because of her influence. This creates a concentration of all the evil aspects of the Doctor's character, which are somehow spun off into a separate being, with all of the Doctor's memories and abilities up to that point, but only a single lifetime.
And what he wants more than anything is to have more lifetimes.
I would like to ask more questions about that mindwipe. It seems to me that this would cover everything that happened between "The Snowmen" and "Hell Bent". So, if the Doctor is confronted with anybody who was involved in those adventures, does that mean he doesn't remember them at all, or anything that occurred? So things like the Boneless, the Sandmen, Rigsy...the entire hiding of Gallifrey...total blank?
I also find it hard to believe that, for all the time Clara spent on the TARDIS, that there would be no visual record of her preserved there.
cheers,
Gabe