Actually there's been a previous mention of another Time Lord, the Corsair, who changed sex when they regenerated (that was in "The Doctor's Wife," IIRC.)
As far as I'm concerned, they can cast anyone of any race, ethnicity, age, or gender as the Doctor, so long as they can pull off that essential Doctorness. I think that if Time Lords can change biological sex if they want to, then their concept of gender is probably nothing like the human conception of gender, and it doesn't make sense to say that they 'think of themselves as male' or 'think of themselves as female.' Their society might recognize half a dozen 'genders,' or none.
On the other hand, I'd also be very happy if they brought back Romana or Susan or the Rani or even Jenny, so that we'd have a mostly-female-identified Time Lord around.
no subject
As far as I'm concerned, they can cast anyone of any race, ethnicity, age, or gender as the Doctor, so long as they can pull off that essential Doctorness. I think that if Time Lords can change biological sex if they want to, then their concept of gender is probably nothing like the human conception of gender, and it doesn't make sense to say that they 'think of themselves as male' or 'think of themselves as female.' Their society might recognize half a dozen 'genders,' or none.
On the other hand, I'd also be very happy if they brought back Romana or Susan or the Rani or even Jenny, so that we'd have a mostly-female-identified Time Lord around.