I would prefer (and suspect they have) create(d) an original character for the film. Kahn worked well for the second film, but that was because he had a motive for revenge. Bringing him in would be akin to retelling "Space Seed," which was an okay episode, but, not one of my favorites.
Besides, there's a rainbow of possibilities that can be tapped into from the reboot: the destruction of Vulcan, the instability of the space-time continuum, those new and interesting Klingons, and a whole pantheon of alien species for Abrams & Co to interpret, and an infinite amount that can be created.
On a side note, has anyone been following the comics? The writers admitted that they will serve as a bridge to this film.