Were I in charge, I'd do something similar: a whole season of Clara and Alternate-Eight, who is aware that he should not exist and will cease to be once things are fixed, jumping back and forth through time to restore the Doctor's timestream to it's previously undamaged (well, less damaged) state. The season would end with alternate-8 and Clara completing the job, alt-8 bowing out of the picture as the timeline snaps back into it's proper state, and Clara finding herself back with a restored 11th.
Then, were Matt interested in continuing on for more Who after that hiatus, he could do just that, and if not, then regeneration into the 12th during a special. OR...I think it'd be an amazing fake-out to have The Doctor surprisingly regenerate during the first episode of a new season. That way would probably make it much easier to mislead fans about the casting of a new Doctor.
/fanfic