The ability to treat RVZ and CHD files the same way it treats .zip, .7z, and .rar files at least for the dat files that don't already have CHD files in them like Mame does. I understand that it supports CHD files if the dat actually explicitly mentions them, but not what I am describing.
Don't mix the representation of a set and its content. A disk element in a datfile (or a rom or sample) is the content of a set. A set is a collection of roms and/or samples and/or disks. A set can be represented either decompressed as a rompath subfolder or it can be a zip/7z/rar archive. A disk within a set is currently represented as a CHD file.
It's not planned to support RVZ. Neither as a new set archive format besides 7z/zip/rar nor as an alternative for representing disk elements within sets.