A while back i was toying with using camera raw photos inside MC and had problems with the "As shot" settings in the files. Sidecar .xmp files existed whcih had the correct info in them, but MC can't currently use them.
This, and the fact that i lost a lot of tag info on some movie and m4a files, while moving all my files around recently (my fault though), got my thinking about the side car files idea.
Why not give all files which MC can't currently write tag info to, sidecar .xmp files? It's an open standard, understood by most high end photo organisation software, and given the necessary integration in MC (ie. "apply to sidecar files too" tick boxes in all move/rename/organization tools) would even mean that tag info could be salvaged for files that accidentally get disconnected from the MC library.
For example, if whole folders get moved around in explorer (outside of MC), then the sidecar files would still be there next to the files, and so even if MC disconnected those media files during the move process and then remiported them, it would still be able to read the sidecar files and retain all of the relevant tag info.
Off the top of my head, these files could be applied to m4a, wav, avi, raw photo formats, documents etc.
Another idea which could work, would be to put the sidecar files in one directory like the cover art folder works now. Then when you rename media files, MC would rename the sidecar files to match, just like it does now with cover art. I personally like the idea of big cache folders, rather than files thrown around everywhere.