Yesterday's theoretical ramblings dealt with, let's talk candidates. As the title suggests my female Doctor fantasy cast is Amber Benson. Oh, don't get me wrong, I wouldn't say no to Helen Mirren-- Ah, no, wait, that came out wrong! Well, a little. (*adjusts collar, takes a deep breath, returns to the point*) Anyway, as the title suggests ideal female Doctor fantasy cast is Amber Benson.
Now, my main exposure to Benson's work was as Tara in Buffy The Vampire Slayer but I've seen bits and pieces of her other work, both later and earlier so I know she's got range. What's more, in Buffy she demonstrated a particular skill I think she'd be able to play on as the Doctor: she is very good at seeming small.
Seriously, Amber Benson is a tall woman but watch most any episode of Buffy and you'd swear she was about the same height if not shorter than Sarah Michelle Gellar, who is not a big woman (which was sort of the point). Throughout her time on the series Benson is kept on the peripheral of scenes, a good visual metaphor for Tara's self-confidence issues and when she is the main focus of the camera it's so she can do something important.
This is all very similar to how Patrick Troughton's Doctor was shot in a lot of his surviving material. That's how I imagine Benson's Doctor would be: hiding in the corners of the worlds she visited, leaping to the foreground in moments of crisis. I also have to admit that the photo I found to head this post looks like a pretty good prototype for her Doctor costume: bit mismatched, sleeves don't quite fit, swap in some historic band t-shirts to match the eras she's visiting, it could work.