I would imagine that it would require assigning some kind of masculine-feminine value to every character creation trait
Masculine body > presuming male
Feminime body > presuming female
Beard - overwrites body > presuming male (maybe except Dwarves)
What other traits would you concider?
Being trans involves more than just the possibility of being misgendered, and from a writing standpoint having that be the only time a character's transness is recognized would just be disappointing.
I cant honestly imagine anything else ...
But question:
If you would find that disapointing, you surely find curent state, when your character gender identity is stated outside of the world ... and whole world knows, accepts, and calls you acordingly ... at best equaly disapointing, right?
