You could be a little bit opinionated and limit the choice of formats to a small set of ones that are a good idea. That would encourage good design and it would

You could be a little bit opinionated and limit the choice of formats to a small set of ones that are a good idea. That would encourage good design and it would allow you to shrink the metadata. Perhaps one byte would suffice. It's not necessary to support every algorithm/length under the sun.