Firstly, this is Doctor Who, so never say never to a multitude of possibilities including - but not limited to - the return of Gomez and/or Simm at any point in the future. That's unlikely to happen immediately; Chibnall will want to find his feet with his new Doctor first, and it seems as though the end of the Christmas special really will give him an entirely clean slate to work with, both in terms of cast and the landscape of the show. But The Master has reappeared frequently throughout Doctor Who lore, with little-to-no explanation for how he has regenerated. In recent years, it's been a little more accountable, but even so, it was never fully explained why or how Simm's Master had returned or how he and Missy are able to exist in the same space, as one, and yet have differing personalities and act independently of one another.
As both died, there was maniacal laughter from each; a moment that was both eerie and sad, certainly on Missy's part. Alone, friendless, and without the Doctor ever knowing she decided to stand with him after all, Missy was left to die. But was her laughter the sign of something else? A knowledge, perhaps, that she will regenerate in some way? Has she outwitted her predecessor at the very last second? The Time Lord council granted a new regeneration cycle to the Doctor, and if they're convinced of Missy's desire to work for the greater good, they might just be persuaded to grant her a similar gift - especially considering they already deemed Simm's Master worthy of help.
Another aspect to consider, is that during 'The Doctor Falls,' when The Master asked Missy if she is "the next one along," Missy was uncertain. The Master assumes she's the next one (even asking for advice about being a woman) but he doesn't know for sure. Theoretically, that could mean that there have been other incarnations of the Master in between him and Missy, so as he returns to his TARDIS, which Missy had given him enough time to do, he could regenerate into one of those versions, and it's likely to be an evil one. Of course, as we've seen with similar Doctors before, two of the same Time Lords (in this case the Master) occupying the same place in the same time can lead to hazy memories - which could explain why Missy can't remember her regeneration. In that case, it'd be a clean closed loop - but there's plenty of wiggle room left for Chibnall to insert his own take on the character between the two.
So, while technically this should be the end of Missy and the Master, it's unlikely it actually will be. Most likely, we will see another actor or actress in the role further down the line - and Chibnall will find his own way to explain the character's return. Gomez and Simm will be missed, but just as Simm matched to David Tennant's Doctor, so Gomez was perfect for Capaldi. That closure should allow Chibnall the potential to make a perfect match with his new Doctor, when that person is revealed.
Doctor Who will return to BBC and BBC America this Christmas.