Lots of excellent suggestions for the mature Dr. Strange. Most of these guys have the comic book look.
It does seem practically inevitable they'll go for an origin story, so they'll need someone younger than the mature Strange. And sorry, I think handling the rather trite reluctant hero story is going to call for a higher level of acting skill than an established hero.
And I still think Strange is not intrinsically white. White guy gets oriental powers was done by The Shadow. They tried to dust off the narrow-mindedness in that movie, but it is going to be a problem.