We already saw them... Romana... The Rani... in the books there is also mentions of other female Time Lords\Ladies... Leela and Ace also became Time Ladies, and The Doctor had a granddaughter back in the early days, even though Time-Lordiness was not well developed back then. Her name was Arkytior and when posing as an human she goes by Susan Foreman. Jenny is a female clone of The Doctor (and also a weird case of "Cast Incest"), and River Song is actually an human that was conceived in the TARDIS, but still has Time Lady\Tardis powers.
The Doctor also said that Corsair was also a Girl a couple of times. We will probably see a woman Time Lord in the future, and Russel T. Davies will make it happen in some sort of strange and complicated way. So yes, there is a chance that The Doctor will regenerate as a girl. I'd also like to see the whole "River Song" thing sorted out.
Well, keep it cool Whovians!
It is not likely since the Doctor is the last of the Timelords. If Rommana was not summoned back to Galifrey for the war, she could still be alive in E space. There would be a chance of them meeting up.
They could bring Jenny back.
I don't think it's possible for the Doctor to regenerate as a female. I know the writers can do anything, but that just seems wrong.
We HAVE seen female time lords before... if you don't remember Leela (cause she was in the series back in the 60's or 70's or somethin) there was also those two women who were sitting with Rassilon at the table on Gallifrey in 'The End Of Time, PLUS what about Jenny and River Song? They're PART Time Lord...I guess
We've already seen female timelords: Jenny, ones seen in the original series. The Doctor stated that there were female timelords when Donna asked if Jenny should be called a timelady. There were female characters visible on Gallifrey in TDOTD. Hopefully we'll see more in future. Maybe one will become a more major character.
However, I don't think they'd ever consider the idea of making a future Doctor regeneration be female. It'd be too controversial. It'd alienate audience members.