Zipping an HTML file and the resources is not equivalent, since the browser behavior very much depends on HTTP headers etc, which MHTML doesn't capture. From the existing formats, that leaves pretty much just WARC, but that's also already kind of niche, has a a bunch of interoperability issues in practice, would probably need extending/reinterpreting to fit, and just like MHTML uses base64 for binary files, which is needlessly inefficient for the purpose here.

The more important things to specify are the browser behaviors when working with such files and the signature mechanisms, and those specifications you'd also need if you reused a format.

AFAIK bundles are designed to enable Google's CDN to serve up signed bundles from their servers and masquerade the bundles as coming from your site's domain. That's not something MHT files were designed for. (This actual feature is separate.)