It would be great for this to start with a survey of open data and open definitions in the wild.
On open data: Who is claiming to be open data? What challenges have they faced? What legal approaches do they use? Are there obvious gaps of data that should be open but isn’t for some reason?
On open definitions: what other ones are there? How many are actively maintained/supported? What are the most successful ones?
That’s something it’d be great for OKFN to bring some organizational resources to bear on, so that these other questions are not asked in a theoretical vacuum.