Despite the best efforts of the snow, I managed to get back last night after attending Dragonmeet in London. Among other people, I got to geek out at C7's Angus and Dom and Andrew Peregrine, went "ooh" when they announced that Gar Hanrahan is in the process of finishing a Primeval RPG adapting the Who system (I imagine Talking won't be the most favoured action in the initiative order in this version, and Gar also mentioned a system for building up paradoxes until time changes) and got to talk a bit about the Doctor box sets.
While I was away, Siskoid's Blog Roundup linked to a very kind review of A Series-ful Of Plots.
And since you come here for ideas rather than bloggery, the sudden massive snowfall makes me think...
The travellers return to present-day Earth after adventures in the past and future, to find Britain (and much of northern Europe) buried under two feet of snow. Services are grinding to a halt, UNIT doesn't have enough snow tyres... And there is talk of strange figures in the snow. Footprints are soon filled in by more snow and lose their shape, but they're large and deep, suggesting something big and heavy is abroad in the frozen land. Could this connect to the Pharos Institute reading strange energy fluctuations from a series of weather monitoring stations around the country?
Ice Warriors of course (see Midwinter in "A Series!") or maybe Abominable Snowmen, or anything that wants to disrupt things and doesn't mind the weather. And the wider madder Whoniverse being what it is, undead Vikings bringing about Fimbulwinter are just as likely.