IF you had to pick one, which gender should the next Doctor be?

[True Federalist (진정한 연방 주의자)]

Trelph.

(From "The Trelph is a Solitary Creature", a novella in the December 7, 1981 issue of Analog. In the story, the trelph is a third gender of a fish like creature on an alien world whose function is to regulate when the males and females of the species go into heat. Why a species would have such a third species was never explored, it was just a plot point.