If Moffat wanted to do the 13th doctor idea and avoid the Metacrisis issue he could do a '2 minute' Doctor'. What I mean by that, is if the 11th was in a battle and regenerated to someone other than Capaldi. We get that 12th Doctor for a couple minutes then he gets injured again and regenerates to Capaldi. After all, Moffat did write 'Curse of the Fatal Death' (where something similar happened) and does like to re-uses his ideas! LOL Of course, he already played his 'unknown' Doctor card with the War Doctor.