Great questions/thoughts Dan, and some overlap with recent fascinations of my own as well.
The email-as-iMessage is the perfect analogy for what email should be. One of the greatest benefits from such an approach is making email more secure, smart, and ephemeral, like WhatsApp exchanges or Slack channels that automatically expire threads every day or month — and integrating functions from third-parties within messages. I suppose some would say Slack among others are tackling this, but agree the backwards compatibility to accommodate all email is key.
On the use case of blockchain for “collectibles” that we are seeing now — I wonder how it can be applied to the authentication and ownership of digital art (has yet to really accrue value given ease of reproducibility and the king makers of art world not taking it seriously), but I also wonder if the tech can extend itself as a source of truth for ownership/authenticity of real-world valuable assets like art (combatting forgeries), real-estate titles (negating need for title insurance). Exciting ideas from an unlikely place (virtual collectible cats).