The 50th special has been completed and we know that the Zygons will be returning.Allegedly, there is a another, even bigger enemy lined up for the 50th special and could be the main villain of the celebratory episode (possibly). This spoiler has apparently come from a number of very reliable sources and told to Doctor Who Spoilers who have confidently confirm this spoiler to be true.
The returning enemy is:
Omega was a great intergalactic engineer and co-founder of Time Lord society, but circumstances bent his mind so that he threatened the entire universe. He was the only person ever to live within the anti-matter universe, which he subsequently ruled over.
No one has been confirmed to be playing this character but maybe they are hidden in plain sight? In the Arc of Infinity Omega used the Doctor’s biodata (in this case the 5th Doctor Peter Davison) to take physical form.