LOL! I've seen some pretty interesting 10.5 fics around -- including one where he turned out pysically male but *completely* gay, thanks to Donna's mental influence, thus adding some serious complications to his and Rose's supposed "happily-ever-after" relationship. Heck, *I* wrote some crack!fic where 10.5 and Donna end up fighting over Captain Jack, since now they *both* think he's teh hawt.
However, it's true that I *havent* seen anyone address a possible physical-gender crossover; most stories work with mental gender-identity switcheroos. (Which isn't to say that physical gender-switch fanfic doesn't exist, I just haven't seen it.) I think most folks base that on the way that Donna *doesn't* go into hysterics after getting an eyeful of naked!Doc in TSE . . . which implies that he looked pretty normal, externally. 'Course you could also argue that Donna didn't look that *closely* . . . ;)
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However, it's true that I *havent* seen anyone address a possible physical-gender crossover; most stories work with mental gender-identity switcheroos. (Which isn't to say that physical gender-switch fanfic doesn't exist, I just haven't seen it.) I think most folks base that on the way that Donna *doesn't* go into hysterics after getting an eyeful of naked!Doc in TSE . . . which implies that he looked pretty normal, externally. 'Course you could also argue that Donna didn't look that *closely* . . . ;)