Thought is the doctor was gone in most of The Tenth Planet and kinda wanders in at the end saying "I'm feeling a bit frail" or whatever the line is. So the thinking is while he was gone he runs into Capaldi's doctor and they have an adventure while the companions do their thing in the original story. So both regenerate from it. This is from the rumor the first doctor will be back at some point too. Could be so wrong, but I would like it as one last Moffatt timey wimey story.
Based on the finale (at least until Christmas), it looks like you're very likely right. They're going to use David Bradley to play the First Doctor. I watched the 2013 An Adventure in Space and Time and I thought he was really great in it as William Hartnell.
I did mostly enjoy the two-parter finale, but I'm afraid they're going with the hard turnover like with RTD to Moffat. I did not like the change from Series 4 (my personal favourite) to Series 5. It looks like they're going to change the Doctor, companion(s), and the Master. I wish they would make these show runner changes easier on the big fans. I would have at least liked to see Bill as companion for the next Doctor, not to mention have Missy return (although I suppose the latter is definitely possible again as we now know that Simm's Master regenerates into Missy). Even so though, there's no way the Master is dead for good. Missy's apparent death seemed too convenient for the plot. She'll either appear as she has or she'll be back after another regeneration.
Michelle Gomez is done as the master (she said she wanted to leave since Capaldi was done) so while I imagine she would be happy to return in the future with a guest appearance we'll be getting a new master. I personally don't mind the hard turnovers but I just wish we would see a doctor last longer than three series.
Also sad that Moffat is leaving but change is what makes the show and I'm looking forward to see what Chibby can do/ a new doctor and companions.
