Imagine person A of your OTP is a Time Lord who brings their companion to visit person B, whose child is destined to be famous. However, when A gets there, they find that person B is dating the wrong person, and they have had no children. Later, when person A regenerates, they find that they look exactly like who Person B is supposed to be with. They then have to go back to B’s time to have a child with them and make sure history goes as it should. (OT3 bonus: Person B was originally dating C).