There a few really outstanding actresses that you would sincerely wish to see as part of the cast in the current rendition of Dr Who. For my money, Keeley Hawes is one of that select company.
The BBC recently announced that she will be joining the series, in order to play the villainous Mrs Delphox - an evil banker (a bit of redundancy there, I think)!. Hawes has played a series of amazingly strong female characters, from the MI-5 case officer Zoe Reynolds ('Spooks') to the elegant Lady Agnes in the recently cancelled (well, not renewed, anyway) 'Upstairs Downstairs'. However, my favorite was her portrayal of the complex character of Detective Inspector Alex Drake - opposite a gritty, aggressive Philip Glenister as Detective Chief Inspector Gene Hunt - in the late, lamented, 'Ashes to Ashes' crime thriller.
I think Peter Capaldi as the new Doctor had better be on his mettle, as Hawes is quite capable of stealing any scene she is in. Rumors about future leading roles for females in the show have flooded the message boards of various Dr Who sites since the Dr Who 'showrunner', Steven Moffat, dropped hints that the Master will be returning during the forthcoming series. Who knows, could we yet see the first female Master regeneration? Now that would be a meaty role for any actress.
As far as I'm concerned this casting decision adds to the solid appeal of the franchise, and reflects its consistent ability to attract top-flight acting talent. The selection of Keeley Hawes also indicates the continuing drift away from the original 'early teen' audience demographic, towards a more mature show.
One interesting piece of Dr Who trivia; in the last series of 'Upstairs Downstairs' Keeley Hawes played alongside another Dr Who fan favorite - none other than Alex Kingston. I certainly hope 'River Song' has passed along some tips!