If the forsaken became a third faction because sylvannas leaves/gets kicked from the horde.
Well of course the already forsaken in the horde become the type who stay in the horde and follow the warchiefs ways.
While sylvannas is doing her own thing, such as killing people to resurrect them as her own?
We can already say she's become her own type of lich king, I wouldn't be surprised if she does something outrageous at the end of MoP.