expand the Bridge keywords feature

That would be up to the file type (and their source applications) to support XMP. XMP is the metadata interchange standard that Adobe applications read and write into their files. Other file types also support XMP, such as JPEG and PDF.

Other applications, especially Camera RAW processors, tend to store metadata in any number of sidecar XML files. In the case of Office XML docs, they have metadata files buried in the package that represents the document, but the documents are really ZIP archives.

Applying tags or keywords to ANY file is precisely why we use Digital Asset Management systems, which often separate the metadata from the binary. Even files that don't support XMP can be classified in a DAM.

Many file types are proprietary or the standard is old and not designed to hold arbitrary text fields.

I'd love to see Adobe even support platform features, such as MacOS tags/labels.