We've discussed this before and the basic idea is pretty much the same, unless your Christmas Special is also your Series Finale.
Do something different from last year, and from the rest of the run. A special guest PC or two wouldn't go amiss, particularly if PCs left in the series finale but their players want to bring in someone new next series.
Here are several more ideas that you might consider.
And an actual play example.
X: I'm the Ghost of Christmas Past...
The special could be key to the series, or have nothing to do with anything.
If in doubt, you can always bring back a popular monster we haven't seen for a series or two. At Christmas.
Aim for at least a bit of heartwarming. Even if it's delayed until the New Year Special.
And don't be afraid to go mad, even by the show's standards. Christmas is a time for killer Santas, TARDIS car chases, unconvincing Queen stand-ins, frozen hearts melting and flying sharks.
Example: I Saw Three Ships Coming Sailing In
The Doctor and Emily, intending to visit Richard and his brother for Christmas, land a long way from their intended arrival point, on an orbiting platform in deep space, at a peace conference between a dozen intergalactic powers. Can they stop an assassin plunging the galaxy into an apocalyptic war?