Hugh Laurie would make a freaking awesome Master or Valeyard! But no beard. No beard. If he wasn't already in the 50th, John Hurt could have been a good choice.
This is just a little out there... But what about Colin Baker? "What! His face! Why the hell do I have HIS FACE!? Am I subconsciously trying to send myself a message? NEVER! I must have HIS FACE because I am superior to him in every way!" In ancient Rome, there was only 15 names (for the upper classes I'm assuming?), which were arranged into different combinations of 4 of those names to delineate family, position and identity. Maybe there is a serious limit of faces if Regeneration is much more artificial than we have been lead to be believe?
Well, not exactly. They had a very limited list of praenomina (first names), but not so short. Then they had nomina (clan names), and cognomina (started as nicknames to distinguish people with the same praenomen and nomen, but ended up being inherited as family names inside a clan). So later on the added agnomina, to distinguish people with the same preanomen, nomen, and cognomen.
What about Steve Coogan? He's always stood out to me since he played Hades in the Percy Jackson adaptation. As lukewarm as the movie was I thought he had the most entertaining parts.
I don't think the Master would realise he had The Doctor's face if Colin played him, given how much he's changed since 1986. Not that I would want Colin to play him.
Much as I wanted Alex Siddig to play the Doctor, I think he'd make a great Master too, although casting a black/asian actor or a woman would provoke cries of "Oh not good enough to be the Doctor but they can be the bad guy" although personally I think it would be a good intro into the 13th Doctor maybe being of ethnic origin.
If Capaldi uses his own accent, and I think he will, I could see the BBC getting a bit poncey about The Master having another non-English accent (or 'regional accent', in that horrible expression). But if they're not, I think James Nesbitt would play well opposite him. And Jekyll showed that he makes a great Moffatt-penned bad guy. He and Capaldi once appeared together in a BBC drama called Passer-By. Nesbitt was a witness to a crime and Capaldi was the barrister cross-examining him, a fairly small role. IIRC, they did seem to have quite a good dynamic together in that short exchange.
That would confuse many newer fans, and even old ones alike. Why not just bring back a female Rani if we need a female rogue timelord? I think that was the whole reason she was even created.
I really like the idea of Dourif or Waltz. Both are great in whatever they do. Slightly OT, is there any kind of role for Brian Blessed? Obviously, not a small role, because I don't think he's one for small roles. But a one-shot?
Maybe Donna's Marriage didn't work out, and we could bring Brian Blessed in to play a Warlord and become a great husband to Donna...Oh wait....we've done that already (On Classic Who and on Blake's 7) I'm always up for Brian Blessed Guest Starring Roles