As those of us who have been around long enough know well, the Au Ra as we know them today were previously conceived in very different forms. Compared to their original concept artwork, they've been largely defanged and an unfortunate case of vast differences between the models for the two genders has left them hardly looking like they belong to the same race in respect to body size and proportion.
With 6.5's finale, it is not unreasonable to suggest that history may repeat itself with a future void themed expansion. It is also not unreasonable to suggest that in the same vein as other MMORPGs, that FFXIV will continue to see class additions as well as playable race additions, as these are staple features for selling expansions.
Interest in the features Au Ra originally had, such as more demonic appearances, different horn shapes, as well as the female model having a build more in line with their male counterparts have often been requested in the past.
I can think of no better point to introduce this as a separate playable race in the future than when the story, inevitably, pivots back to the Void. As of now, there are very little other plot threads that could reasonably carry the main story and with an expansion set in the Void, a race of full-on Voidsent or half-Voidsent would be an excellent and almost expected addition. It wouldn't do for say a Legion-like expansion to release without Demon Hunter, etc.
I propose using the highlander male build with an edited slimmer waist for the male model, and using the female Viera model for the women as it is athletic and tall enough to go with the former. I believe that in addition to horns being a separate, customizable option in lieu of tails, that various beard and facial hair styles should be made available for the male faces. As lore changes on the fly - as it did for Viera and Hrothgar both in order to accommodate their missing genders - I see no reason why an explanation for a playable Void themed race cannot be found.