Ok, so my theories on the Christmas Day episode:
1. The woman in the trailer is someone from the Doctor's past, personally, I'd hope it was the Rani. Or another Time Lord, there have been a lot of Omega symbols with the army since Series 6. I kinda hope that the woman is the female Omega
2. We've seen the alliance before in the end of Series 5, they were joined by a few more of the old favourites then, but I reckon that's who we're going to see.
3. The Doctor will be forced to regenerate, rather than willingly do it. I only think this because of the trailer when Clara says help him change the future (ref to Name of the Doctor).
Also, question, surely if the Doctor stops himself "falling" on Trenzalore, then does The Name of the Doctor still stand as a canonical episode? Because technically it would have been rewritten. UNLESS, during the Time of the Doctor they jump forward in time to post NoTD and fix the Tardis. Who knows, we'll find out in less than 24 hours