Since Steven Moffat dropped into the Doctor Who producer's chair in 2010 the Christmas specials have featured a common theme. Last year's The Time of the Doctor aside they've all been heavily inspired by existing works (and it could be argued that Time of the Doctor was inspired by a certain kind of fanfic). 2010 was inspired by A Christmas Carol (and even nabbed the title). 2011 was The Doctor, the Widow and the Wardrobe, an homage to The Lion, the Witch and the Wardrobe so logical it's amazing it took 48 years to do. 2012 went for Victoriana in general, even though The Wizard of Oz would have sat nicely with Moff’s established theme with the TARDIS taking the place of the death-dealing falling house.
The approach hasn't had the best results. His first stab at the tricky to nail Crimbo standalone, which is intended to be watched by an audience that's half cut and likely to doze off, was about an old man who kept his girlfriend locked in a freezer and only let her out once a year, with her last hours implied to be spent underneath a groaning Michael Gambon. It’s not what you’d call a heart-warming tale. It also featured Moffat's irritating obsession with writing time travel into his plots, as opposed to using it as the impetus for a story (there's quite a difference).
His second attempt quickly dropped its promising source material and told us a story about a tree king and living baubles. That Bill Bailey's minor cameo was one of the best things about it says all that needs saying. Meanwhile the 2012 melange was more worried about setting Clara up as the impossible girl than striving for the realms of good or even memorable, a poor decision for what should have been a Romp™.
Judging by this year's trailer I think Moffat's changed his sources of inspiration. What's been released shows a bunch of people wandering around a sci-fi base or ship, some aliens hatching, and Father Christmas. There's also a shot of (presumably) people with cloths draped over them sitting up on gurneys. That clip and the alien hatching make me think it's going to be a survival story in which people are picked off and converted one by one. Specifically it looks like it's been influenced pretty strongly by Alien, its sequels, and its rip-offs. Just what you want on Christmas Day.
In fairness if this is what Moffat's written it's not the worst idea in the world. An homage to action films, a genre which doesn't generally require much attention from the audience, is a good approach for a Doctor Who Christmas episode. It allows for a story where there's a threat (something Moffat scripts all too frequently lack) but in a format where people can follow along by knowing the basics of the genre.
The main reason I've written all of this is that I have a prediction on the alien race in the episode. Assuming it is a story about a small group of humans getting picked off or converted by an alien in space there's a natural race to bring back and use in the alien role: the Wirrn.
It could be interesting to see them used again, especially when you take into account the advances prosthetics have made since 1975. But it would also be a risk for Moffat. The Ark in Space is a highly regarded story by a highly regarded writer that kicked off what is probably the most highly regarded era of the show. This newer episode would be open to a host of comparisons and would need to be very well made in order to not come off looking like a nicely made but poor imitation and an ill-advised, unnecessary remake. It's possible the modern production team could better The Ark in Space but doing so would hinge on a top notch Moffat script. And he's not had one of those in a while.