> They don’t standardize encodings, revisions, metadata, or even how filenames or paths are represented!

Sure, but some unified diffs, e.g. the ones produced by git, are quite regular. It's also common practice to express diffs as RFC822 email messages (often because they come that way), with headers and descriptive text.

I can't see DiffX getting traction. It's too alien. Too divorced from present practice, no matter how theoretically robust. It's like XHTML2.

Solving the same problems, I'd just establish conventions for sticking the needed metadata in RFC822-style pseudo-headers above the diff. This approach would work with, not against, existing tooling.

Not everything needs to be JSON.