I'd love to see a the option to select one of multiple available word lists for passphrase generation. The reason being that I am not a native English speaker and I'd like to create passphrases with words that I am most familiar with. I don't expect Proton to manage word lists for every language. A great option would be a "bring your own word list" feature. Then I could simply upload e.g. the DeReKo word list and use it for passphrase generation.
I would be fine with disclaimers that Proton cannot verify the quality of self curated word lists and therefore cannot ensure that they can generate enough entropy for for high quality features or similar, if this is a concern.