I really wish there was one that's already built consensus. I've asked about this on reddit, in meat space, here on lemmy, and other places with not even a front runner for a widely accepted option.
The closest I've seen is in socialist spaces, where comrade replaces all other honorifics.
There's great value in the formality of honorifics, and having a gender neutral option would go a long way to cracking one of the language barriers. It may seem like small potatoes, but not having a formal honorific is as big an issue as the struggle for the singular they/them. It really would help tip thinking towards non binary and gender neutral people as being respectable (by people that find it easy to dismiss them).
It's fine if individuals don't want honorifics at all for themselves, but there's a need for it for general use.