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.