No way they'll do a woman... I just don't see it.
Why? In 50 years they've never had a woman, so they shouldn't have one now? There's something in regeneration that prohibits a sex change? If that was the case how come in all the incarnations that I've seen (Tennant and Smith), one of the first things they do is look in the mirror and be glad they were in human form and not something else? Maybe I just don't know how Regeneration works, but if you're changing into another body or form, it shouldn't be conducive to one kind, should it?
It was stated in The Doctor's Wife that gender can change with regeneration. So, no there's no IN story reason that it can't happen.
But, I think Klaus was saying they won't make that choice. TV success breeds the desire to NOT take risks. And casting the Doctor as a woman... I think for many they would see that as a risk.