Since Windows and MacOS don't seem interested in having mutually supported advanced filesystems, it would be nice if we just standardized on a flat file structure to implement portable features.
fat32 is very portable but has a 4GB limit and lacks entered attributes. exfat probably supports everything but until very recent open source drivers it lacked cross platform compatibility.
Plus too many devices are going to be limited to fat32 so we might as well just use that as a common denominator.
It would be nice if a DS_Store system could be used to associate split files (e.g. $NAME.1 $NAME.2, etc.) as a word around for the 4GB limit. User permission attributes and other extended attributes could be stored as well.
What's interesting is that Fat32 had this feature implemented on OS/2 and Win NT via files with the "␠EA.␠SF" suffix. Unfortunately looks like it was dropped in Windows 2000.