According to Sean McGrath, aliens don't use XML. He says they have separate technology stacks for dealing with tabular data, written text data, and relational data. I wonder, then, what the aliens do when they want to mix their data? :)
Perhaps a non-XML alternative for data representation will reemerge at the cutting edge some time in the future, but the homogeny issues will still have to be addressed by this new definition. CSV++ would have to find a way to embed or uniformly refer to XHTML++ and N3++ data. XHTML++ and N3++ would need simllar embedding.
XML with namespaces looks like holding the top spot in being able to both define the structure and identify the correct interpretation of its content for the time being.