There’s quite a bit of activity currently happening on MADS (Metadata Authority Description Schema), not to be confused with MODS. The basic idea of MADS is to represent MARC information in XML format. There’s currently a MADS 2.0 draft schema out for discussion.
There’s also a MARCXML schema and a MARC to MODS mapping. If you want to understand the shades of difference of purpose in these different mappings, you’ll have to ask a metadata specialist, such as my colleague Robin Wendler. I’m just a software developer who works for a library.