There are a few fandoms I follow where I see people arguing why certain forms of diversity are or aren’t possible. Two in particular always stand out for me:
In Warhammer 40,000 fandom, people argue the biological feasibility of female Space Marines.
Fans of Doctor Who argue over whether there’s precedent that would allow the Doctor to regenerate as a woman.
In both of these cases, my answer is the same: the creators don’t need to justify it. They should just do it.
(This isn’t about the constraints being fantastical, either. If you try to offer me a “can’t” about, say, race in a historical piece, I’ll point you at Medieval POC and send you on your way.)
Have the Doctor regenerate into Hailey Atwell. Introduce a new edition of Codex: Space Marines with named, unambiguously female Space Marines.
Don’t worry about the how. Just do it.
Because here’s the thing: it’s a lot easier to argue with a hypothetical than with a fait accompli. It’s a lot harder to argue that something “can’t be” when it’s right there on the screen, or right there on the page.
So don’t give the naysayers a hypothetical.
“The Doctor can’t be a woman.” “Really? ‘Cause I just marathoned a season of the Doctor being a woman.”
“There are no female Space Marines.” “Really? ‘Cause my Codex has stats for a Captain Victoria Argentia, and an Olwen the Swift, and a Venerable Proserpina…”
Give them a fait accompli.