Any thoughts about Don Tapscott?
Too much praise probably but might work in combination with sb more down-to-earth.

I'm currently preparing a small proposal for a seminar lecture about blockchains that might be given next year at a local conference (depending on how convincing my proposal is).
The seminar is usually held by 2 persons with different views/backgrounds on the topic and runs for an entire week.

Can you propose me some good lecturers?

The conference organization usually tries to get the leading people of the respective field, although europe-based lecturers might be more realistic.
I was thinking of a setting where one lecturer focuses more on the economical/social impact and the other one on the technical aspects, although audience will not be very techy.