Doctor Who recently ended its ninth series (since the relaunch in 2005), but let's back up to the last series finale. Danny Pink, Clara's boyfriend, died tragically in a car accident, then died heroically as a Cyberman. There was just one problem: this guy.
Orson Pink was heavily implied to be descendant of Danny and Claras' from the far future – he gives Clara a toy soldier, so she might pass it on to her children who would eventually pass it on to him. But how was this possible if Danny was now dead?
'No problem,' some fans replied, 'Clara might be pregnant.' Then series nine rolled around, and nope, Clara could eat all of the soft cheese she wanted.
'Fine, maybe she will travel back in time to meet Danny before he died?' Again, no, Clara died without ever seeing her boyfriend again (at least on screen, and not in a dream).
This left Orson's parentage unresolved, until Clara was saved (or at least given a stay of execution) by Time Lord technology. She was last seen travelling with fellow immortal Ashildr (Maisie Williams) in their own Tardis.
This is the key, accordig to Reddit user Huwage.
For those unaware, in the episode 'Listen', Orson, a future astronaut, was stated to be the many-times great-grandson of Clara and Danny Pink. But later in the series, Danny was killed, and he and Clara never had any children. This renders Orson's very existence impossible, creating a paradox as Clara had already met Orson in the far future. But after 'Hell Bent', this may no longer be the case.
Clara now has effective immortality until she chooses to return and face the Raven on Trap Street. She also has a time machine. What's to stop her travelling back and seeing Danny Pink again before she accepts her death? What's to stop her having a child with him (other than potential questions of biological processes for an effectively dead woman)? In short, what's to stop Clara from finally beginning the evidently long-lived Oswald/Pink dynasty?
As Clara no longer ages, she would be able to slip into her old life while her past self was out and disappear before she disrupted the timeline. A bittersweet reunion, but at least it would solve this nagging question.
Incidentally, Steven Moffat's own proposed explanation for Orson involves great-uncles and is much less romantic.