Cencorship Circumvention Tools like Snowflake which actually work in highly cencored Countries like Russia, Iran and China
Proton has similar to Signal an build in Cencorship Circumvention Tool. But this doesn't work if you live in Countries which don't just block your website on a dns level but use more advanced forms of blocking.
Stealth is a good start, but to really make a difference in these cencored countries more advanced circumvention methods have to be implemented.
Tor uses Brides.
Snowflake is a huge success and works since years in China, Russia and Iran without problems.
Proton should like Tor have Bridges like Snowflake, but for Protons case only allow to visit the domains of the proton services. It must be prevented that the people wo voluntarily provide these Proton Bridges become exit nodes and that their IP Adresses get abused.
If you can only visit "proton.me" and the other Proton Service domains, the IP Adresses can't be abused outside of creating Proton Accounts.
Proton needs an Browser extension for Snowflake (like Tor) to make it extremely easy to be a bridge and contribute to cencorship circumvention.
With Orbot, it's also possible to be a Snowflake Proxy on Android and IOS mobile devices.
Encrypted Client Hello (ECH) also would be extremly helpful and important, altough as time passes, these Countries will block the ECH protocol nevertheless.
Edit: This idea already exists, which I didn't know of, when creating this post.
Here is described how a VPN Provider could use Snowflake to unblock the VPN in cencored countries: https://github.com/WofWca/snowflake-generalized
Forum Thread about it: https://forum.torproject.org/t/snowflake-but-faster-thanks-to-not-using-tor/14433
This idea is already getting implemented and used: https://snowstorm.net/
Signal is right now facing these issues with Russia and Venezuela. Their build in circumvention tool doesn't work, because Cloud Providers like Google, aws and Azure don't support it.
This could become an issue for Proton as well in the near future.