The next version of FIX can try, but everyone stopped upgrading FIX versions years ago. Also, once you standardize the liquidity field, you need to think about its values. These expand continuously...it used to be two or three values, now some systems use a dozen or more.
Basically, if you want a protocol for legal documents, you may as well use an existing one, such as PDF, XLS, TeX, etc. Trying to embed more domain knowledge in the protocol will never work at a highly generalized level. It would be like asking HTTP to standardize e-commerce.