You either go with JSON because everything talks JSON or you go with something that doesn't have an explicit parsing step like flatbuffers or capnproto.

If you don't care about parsing CPU efficiency then gzipped JSON beats protobuffers, CBOR, etc when you care about bytes sent over the wire.

If you care about CPU efficiency then protobuffers, CBOR, etc are worse than flatbuffers or capnproto.

There is not a lot of space for a new standard between these two existing categories.