The (a?) featured threat in the Doctor Who 50th Anniversary Special was leaked today, and it's the long-overdue return of the Zygons! Check out the "redesign".
The Zygons have appeared only once in canon - Terror of the Zygons in 1975, where they fought the Doctor, Sarah Jane, Harry and UNIT - but they've been surprisingly durable in fans' imaginations. David Tennant has said they were his favorite monster, and they've starred in fan-produced videos and audios courtesy of BBV, as well as official audios from Big Finish and novels not just in classic ranges, but in the recent 10th Doctor range as well. They're one of the relatively few classic monsters to currently have their own action figure (of course I have one).
You might think this is still an odd choice for the Anniversary Special, since they have no connection to the very beginnings of Who. The only historical factoid about their one story is that it's the last proper UNIT story to feature the Brigadier, and Harry Sullivan's last appearance, hardly Anniversary material. But I'm kind of expecting the Special to feature MORE than one alien threat, and the Zygons may be only who the 10th Doctor is fighting in his solo bit of the episode. Seeing as they're Tennant's favorites AND they were first encountered at Loch Ness in Scotland, the country Tennant is from, they seem to jokingly tie into HIS particular character.
But with at least one costume made, that would allow future stories to make use of them in full-length episodes...