Executive producer and script writer for Dr Who, Russell T Davies, said he would not rule out reincarnating the Time Lord as a woman.
Mr Davies said he would have the nerve to have a woman playing the role.
And he said it would also be possible to have a Welsh Doctor, such as the actor Michael Sheen, whom he described as brilliant.
Asked if he had a wish list of talent for the show, Mr Davies said: I would love to get a gripping American star like Sigourney Weaver.
You often get an actor who says, I would love to be in it, and then you have to speak to the agent.
He said he initially thought a hand-written letter he received from comedian Peter Kay, begging him for a part in the show, was a joke.
It said Yours sincerely, Peter Kay. PS garlic bread.
An episode of the new series has been written by Matthew Graham, co-creator of the hit BBC show Life on Mars.
Stephen Fry has also been pencilled in to contribute to the third series, Mr Davies said.
Speaking at the launch of the new Dr Who series at the Wales Millennium Centre in Cardiff, the script writer said there was not a deliberate attempt to put over a message, but added that swearing, violence and blood were off the agenda.
We would never have a human shoot another human, for example, he said, unlike in shows such as EastEnders.
The first episode of the new series, which sees specially grown plague carriers in a futuristic research laboratory, had drawn parallels with the subject of animal testing.
Mr Davies said he drew his inspiration from newspapers, grabbing what is happening now and placing it in a science fiction setting.
Dr Who has always tapped into classic British horror, he added.
He said he liked to keep the relationship between the Doctor and his sidekick Rose on its toes and said they were best friends.
A kiss between the pair in the fist episode of the new series is a bit of a gag and there is another kiss with a twist in the fourth episode, he said.
Mr Davies said there was a natural chemistry between David Tennant and Billie Piper, but if the characters they play ever consummated their relationship: I think that would kill it dead.