One of the thing I find absent from the app quality guidelines would be the various forms of accessibility.
There is currently no criteria for availability of translations of the metadata (text and screenshots), which is a very important aspect of making Flathub usable for the vast majority that does not speak English.
I looked for it but saw no way to specify the alt-text for the screenshots in the metainfo file (from what I’m seeing the captions are not meant for that, they don’t show up in the alt-text, and second they’re expected to be short, alt-text is from my understanding expected to be more precise).
One other thing. Since XML is not a very nice format, it’s easy to make mistakes writing it, I find myself wanting to be able to preview the results of the changes to the metainfo file. I know it’s possible to preview the banner with aurea, but I care much more about previewing the entire app page.