I would like to suggest the availability in Proton mail of "dynamic aliases".
By this feature, when a website asks your email, you can enter your regular proton mail email prefix (like "john.martin") followed by a "+" sign, then any keyword you would like.
For example, when John Martin wants to create an account on Amazon site, he would enter "john.martin+amazon@pm.me".
The Proton mail system would detect this dynamic alias and ignore the + sign and the suffix and process the email as "john.martin@pm.me".
This allows to create "on the fly" aliases.
The advantage is that if that alias is spammed, you can detect it and filter it in your mailbox.

Only being able to choose one alias suggestion, and no easy way to customize it are reasons that I never use the suggestion popup for e-mail fields.
Even a simple "customize" or "edit" button on the suggestion that loaded up the extension popup in the "create alias" mode would make me actually click something on it!

Actually, when we want to create an alias through Proton Pass, the suggestions are like :
- visitedwebsite.tld as a prefix
- a few suffixes like @personaldomain.tld or @subdomain.simplelogin.com, or many others like the previous ones

Since many years, I create my aliases as visitedwebsite0123@personaldomain.tld, which seems more convenient to me. 0123 is a random string given by a script I wrote to remember and create my aliases.
So, a great feature would be to be able to personalize the way to create prefixes, with something like :

visitedwebsite([0-9a-z]{4})@mydomain.tld, as an example.