interview for an unknown role, how do I prepare?

Focus on the things you know are relevant:

The company.  What you can learn, what you want to know.

You:  Your background and accomplishments, the career paths that seem likely to come from that, any areas of interest you might want to jump to, or whatever else.

Culture, lifestyle you hope to achieve, etc.

I did an 'exploratory' interview a while back.  I can't say yours would be the same, but mine was all about how I'd fit in generally and what experiences I had that might make me a fit in a certain type of role I'd never done before but had interest in - they had honed in on that interest.  My resume showed I sought out training and experience for it despite never holding the job.

Odds are they do have something, or a couple of somethings, in mind for the right person.  Guessing, though, is just as likely to hurt you as help.  You can't know for sure what they honed in on and how they foresee the pieces falling into place.  So don't try to outsmart the process.  Get all the information you can about their thinking, then prepare to the best of your ability with that information.  Be prepared to talk about "What if"s regarding your career.  But they'll guide things where they want to see them go.  Then you can decide if you like what they see.

No problem.  Let us know how it goes?  And good luck with it.  I know they can wind you tight when you don't even know what it's for, haha.