Become a Matrix Messaging Provider.
I think Proton would benefit by opening up a messaging app that uses the Matrix protocol. Matrix is a federated protocol (like E-Mail), and it's built in the same style as Slack.
If Proton were to become a provider, users who use it will have usernames like, "@username:proton.me", and public rooms would be something like "#public-room:proton.me" on the Matrix Network.
It's entirely open source as well, and there is a server backend built in Golang called Dendrite, which developers might find use for. The main client is Element, which again is built after slack.

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82g74m commented
This would take user experience to the next level. I don't like how sending an email also sends the entire conversation history.
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Mikael Dúi Bolinder commented
I agree. The public Matrix.org server is apparently very slow. You could use Fedora.im, but who knows if they're going to switch to something else. I'm currently self-hosting mine behind Cloudflare, but that's one thing that's now keeping me from re-installing the server whenever I want.