xdg-app looks good, but I think we're still at least a couple of years away from the point where a developer can use xdg-app to reach the majority of desktop Linux users. That's a long time in software development terms.
You're surely right, but to me the important aspect is not only to make a package that run everywhere but also bring the mobile security scheme to desktop ie have apps that are sandboxed and run on top of a well-defined runtime with well-defined APIs.
In that sense Xdg-Apps have a bigger mission that AppImage so maybe we can't really compare them
You can ship xdg-app + your app in the meantime? Consider it a 'polyfill'.
Some best practices around this would be great.