Actually the show needs more evil Time Lords to be exact. And I am not talking about an old mainstay evil Time Lord like The Master either. I like The Master and his arcs on nuWho have been really fun but it is time for some of the criminally ignored, old school evil Time Lords to get their moments in the sun on nuWho.
Who? Omega isn’t just any Time Lord, he is one of the founders of the Time Lord society. he was an overly ambitious genius who, while working on a way to increase the power of the Time Lords, found himself stuck in the anti-matter universe. By force of his own will, Omega was able to gain control of the anti-matter universe and reshape to his own specifications but at the same time that use of power destroyed his body. So Omega was now an extremely powerful disembodied consciousness.
He initially faced off against the 1st, 2nd and 3rd Doctors and it took their combined ingenuity to defeat him. Omega then came back to face the 5th Doctor, even taking over his form at one point before being seemingly destroyed.
As a being of pure energy it would be really simple to explain how Omega is still alive and it would be a great opportunity to bring back not only an extremely cosmically powerful enemy with a legit beef with The Doctor but his ability to enter into other people’s bodies opens up the possibility of getting a really cool actor to play the role and not have him be stuck under that goofy looking mask.
Casting Suggestions: Patrick Stewart (STAR TREK: THE NEXT GENERATION)/Charles Dance (GAME OF THRONES)/Ian McKellan (LORD OF THE RINGS)