The episode ‘The Angels take Manhattan’ left (at least) one thing open: what happened with Mark Williams, Rory’s father, and the Ponds after they got 50 years before they were born. There was an unshot scene by Chris Chibnall that was supposed to clear such thing, but it was put on tape.
Accordingly to this video/cartoon uploaded on Youtube by the BBC team, Rory send a letter to his father explaining why he and Amy will never return from their trip. The guy that delivered the mail was… their son. Their adopted son.