Despite appearances throughout season 8 and a full reveal in episodes "Dark Water" and "Death in Heaven," viewers (and The Doctor) still don't know the circumstances of The Master's regeneration into Missy. The last time Doctor Who fans had seen the character, prior to her reappearance in season 8, was in "The End of Time" where the Master sacrificed himself to save the Doctor and was pulled into the Time-Locked Gallifrey.
Upon resurfacing, Missy is vague on exactly how she survived, and regenerated, leaving a significant amount of time and detail out of her recent backstory. As a result, it's conceivable that Williams is playing a version of Missy from the past - before she grew into Michelle Gomez. Adult Time Lords don't typically regenerate into children but, since fans still don't know the conditions of Missy's gender-swapping transformation either, it's definitely possible the malevolent Time Lord's latest regeneration resulted in a younger new self than normal.
After all, the regeneration process rewrites a Time Lord's DNA entirely - resulting in mental and physical changes, as well as de-aging - so there's no reason that a Time Lord couldn't, technically, regenerate into a much younger body. Given that The Master made the jump from a man to a woman, her younger age would be, comparatively, easy to explain.
Fans were initially divided about Missy, especially those who do not want to see a female Doctor down the line; yet, exploring Missy's twisted love for her fellow Time Lord, long before she resurfaces in "Dark Water," could help flesh out the character, beyond the core twist of a female Master and a great performance from Gomez, with meaningful backstory (enough to win-over some skeptics).
The return of the Master as Missy proves that few characters are ever truly dead in Doctor Who - especially when it comes to characters with Time Lord DNA. River (Alex Kingston) "died" in the season 4 episode "Forest of the Dead" - part-2 of her introductory storyline. Regardless, viewers would see past versions of the character throughout the next three seasons of the show - until her final appearance, when River's disembodied consciousness was summoned to aid the Time Lord in "The Name of the Doctor." In the episode, the pair say their final goodbye and River presumably disappears for good.
While some viewers might feel that the Doctor and River's story had been successfully closed off - especially after Matt Smith (River's main love) passed the torch to Peter Capaldi - a different version of River, one that resurrects the character while establishing a different dynamic to explore with the Twelfth Doctor, could be a welcome mix of old and new.
Understandably, fans would have questions about how exactly River could have taken human form after being "saved" in the library data core but, as with every Doctor Who story, Moffat and his team could easily whip up a timey wimey techno-babble explanation. After all, when the Doctor and River saved the bodiless prisoners of the library (who had been uploaded to the data core), the pair also managed to "rematerialize" all of the inhabitants.
It is possible that someone could eventually figure out a way to rematerialize River as well - even if the resurrection triggered a new regeneration for River.