Self-describing is point-less for serializations. There is a great deal of history here. ASN.1 has self-describing encoding rules such as BER/DER/CER, XER (XML), JER (JSON), and GSER (never mind), and it has non-self-describing serializations like PER (packed encoring rules) and OER (octet encoding rules). XML and JSON are self-describing, naturally. FastInfoSet is a PER-based fast encoding for XML, because it turns out that XML is slow (imagine that). XDR is a non-self-describing serialization format that resembles OER but with 4-octet alignment. Flat buffers is essentially an OER-ish encoding for the same IDL as protobufs, and is much better than protobufs.
It would be nice if the next serialization format either is truly original or just solves problems that somehow none of the many existing schemes do.
How many serialization formats are there? See: https://en.wikipedia.org/wiki/Comparison_of_data-serializati... (which is NOT a complete list).
