Thanks for asking. Your suggestion seems rather extreme, but I'm open to it if you make a good case. I'm curious to see more Tor-first service launch.
1. More blog posts about Tor, how Proton is supporting Tor
2. Updating their Onion link. Their original blog post still refers to a v2 address, which are being retired
3. Supporting Tor broadly for other uses
4. Support anonymous account creation through Tor. I get it, it's hard. Find a solution that doesn't require de-anonymizing users.
As a paying customer, I expect better.
[1] https://protonmail.com/blog/tor-encrypted-email/