I’ve been using Freemind for decades and have a lot of mind-maps in that format. If not Freemind’s XML format then some sort of mind-map format. A markdown type format would be good. I guess you’d just need nested lists?
I’m not suggesting that you have an editing function for it but to be able to view and possibly embed/link-to mind-maps would be awesome!
Anyway, here is the freemind format spec: File format - FreeMind