While it's true that the court T-V blend (use of the plural for figures of authority, just as a monarch would be more likely to answer with "we" rather than "I" in explaining his or her verdict in the same era) has occurred twice in English, it's worth noting that that clear-cut situational use never caused any wide-spread ambiguity. In transition, it modeled perfectly use of "sie" in German. And after the transition, our indexes lost only the equivalent of "du", "vous", or "tú" forms, not any distinct plural.
I also don't see what any of those fantasy elements would have to do with adhering to or exiting social binaries. Until such a binary is an actual story conflict, such that a wizard cannot embrace magic until having blended the apparently female and male halves of its source, or realizing that they're actually the same thing, the two are absolutely irrelevant to each other. No matter how real or fantastic the setting is, unless social binaries are a part, one is not relevant to the other.
And while I certainly wouldn't mind the addition of a third gender option for our simple class flags, unless such a conflict organically fits the story... I don't want it. I have enough issues just with "It's Allagans. It's always Allagans," and "Sinister figures behind literally everything. People can't be 'evil' for normal reasons. Old Gods/Zodiark." I certainly don't need a "Cus n+2 genders" plot-twist forced in there.
On the other hand, give me a race in the Sharlayan Mystlands were believes literally bend reality, changing beastmen between various forms and abilities and, naturally, genders, and I'd probably love the hell of it just because that actually would be consistent: the point there would be how pervasive, how deep, how unlimited, that concept goes. But that wouldn't have anything to do with aiding gender or genderless immersion; it'd have to do with the story of those people in particular.