Either one, really. He just exudes classy English sophistication with a devilish sense of humour, and can do good as well as evil.
Not sure if he'd be too "big" to take over a part like that, but he doesn't seem more famous than McGann or Eccleston really were when they did.
If you must blink, do it now.
John Hurt was huge when he played him, I think Peter Serafinowicz would make either an awesome Master or a great alternative reality Doctor that bumps into our Doctor's TARDIS during some termporal event which they are each studying in their relative realities.
Peter's Doctor would be the Steve Jobs of Doctors with touchscreen everything and all dressed in a classy black suit with turtleneck underneath it with all the answers and our Doctor would be...well...our Doctor (Whoever (s)he is at the time!).
Their realities were identical up until the regeneration and they each look back upon their past with a different take on it all. Each, of course, with their own companions of varying differences and thoughts upon the Doctor(s).
Juan for Show Runner 2019!!! Talk to Bobby, he's going to make it happen!
My bestestest funny evah!!! https://vid.me/ToVD
He's already played one of the best characters to ever appear on Doctor Who.
Reverse the polarity of the neutron flow
CapaldI and Colin Baker were on the show before they were Doctors, and those were live action parts not voice roles like Serafinowicz.
If you must blink, do it now.
The Fisher King wasn't just a "Voice role" it was the height of of Peter's career and he's been quite a few awesome characters already so you know it's important.
Bobby, if you read this you're a regeneration short already!
My bestestest funny evah!!! https://vid.me/ToVD