2. It Will Make The Doctor More Alien
When the Doctor first appeared in 1963, he was traveling with his granddaughter, Susan Foreman (Carole Ann Ford). This illustrates that Time Lords have genders and that they procreate in a similar fashion to humans. Or does it? Perhaps Time Lords are like the Common Reed Frog and can change their sexual organs when necessary. One thing that has been lost in recent years of Doctor Who is how alien the Doctor really is. By changing his gender, the strange nature of the Doctor will be reinforced. The last two iterations can both be described as skinny hipsters with messy hair; theres nothing inherently odd about either of them (unless you consider an affinity for bow ties to be especially strange). Time Lords who can switch genders is unquestionably novel. Imagine a version of your grandfather who is now younger than you and also a woman. Imagine being male and meeting your daughter from the future and having her call you mummy. Whether you agree with the idea or not, its undeniable that the potential to tell interesting stories is there.