Women are regular human beings, they have more than one or two personalities and more than two ways to dress. The male Doctors to date have covererd a wide range of personalities and fashion styles. The writers and the producer would have a range in terms of personality and costume to choose from as broad as with a male actor. Hell, as far as costumes go, they'd even have more choices because a female Doctor can wear skirts and dresses without looking funny to Western eyes. (Jon Pertwee doesn't count.