XList.org
XList is a universal format for representing data in the simple form of a list of items.
Its main purpose is to standardize the way applications and aggregators share and consume lists of items of any kind.
XList is designed to be extensible in order to represent different kinds of data in a standard way.

I never really looked into OPML to deeply but this XList seems to take OPML and extend it to include specifications for different types of media and not just text.