I think there's always been a way to slip a past doctor in if they wanted to.
As an example, there has been a few 'doctor lite' episodes since 2005. It is a chance for some of the crew and main actor(s) to get a bit of a break from a busy filming schedule etc. (Blink, Love and Monsters). So why not have something along the lines of Peter Capaldi's doctor ending one episode finishing off some bad guy, then going back to the tardis and landing somewhere else to find a clue about some baddie that McGann might have met before. He gets a bump on the noggin and in the next time trailer all we see his him and Clara in a prison cell. He's all messed up and telling Clara about the aforementioned bad guy, and how it was a 'few lives ago' since the last time he encountered them. Maybe a little hint of regeneration energy or something just to spice things up?
Then everyone tunes in next week to find out if it really was regen energy, or what the hell it was, and we see the Tardis land somewhere, and Peter is narrating for a minute about when he met this bad guy before the door opens, and...................Paul steps out along with Sherridan Smith. So THAT becomes the doctor lite episode. 40 minutes of Doc 8 and Lucie, before going back for a couple of minutes at the end when it cuts back into Peter telling Clara that was how it all went down last time.. A little hint he might still regenerate before it ends(maybe? Too much?? yes? no?), and then next week Peter and Claa deal with the same bad guy.
Anyway- I've gone on long enough about that- it is just one possibility and I'm sure the writers can come up with something a LOT better.