This does not surprise me. When they said she was in the running I did a search on the subject - more than what I knew, and said "She's likely to get the part given how well she does period pieces."
I'm a little sad - she showed you can be cast in this show and work well along two different actors as the Doctor, but I totally understand the reasoning.
Now, as far as the next companion - I doubt it'll be Alex Kingston as River - unless Jenna leaves midway due to filming for the miniseries and they needed a filler companion off and on for the rest of the series. I *would* say that Maisie Williams except there hasn't even been rumors outside of "DW" that would point this, nor is "Game of Thrones" - I think - ending soon.
Then again - if my wild guess is right, the "Girl who died"/"Woman who lived" is actually two Claras - her DNA was altered in the Doctor's time stream so she became a Timelady, further making her an "Impossible Girl" and Maisie is the next regeneration of Clara.
The fact they have kept this spoiler pretty well hidden though, they may want to try and surprise us this time on the companion.