Once again I was browsing the flagless identities, and found defluogender.
"Defluogender is a xenogender that seems to have floated away, or is lost in some way."
It says there's no known flag for this identity, so I thought I'd try to come up with something.
The blue lines are meant to look like the sky, which is very vast. The balloon is there to represent the gender having floated away, and because balloons tend to get "lost" in the sky a lot.
Hm, what do you folks think?