It would be nice to have the option to do custom HTML/CSS when sending an email. Most other email clients don't have this feature or have their limits on a feature like this, and so there is A LOT of potential for Proton to easily compete in this category.

Proton currently has an option to switch over to plain text, but that seems more like a notepad/wordpad thing than a place for power users who would like to custom-write HTML for their emails. In fact, the plain text feature that currently exists feels like what I'd expect back in the early 2000s for an email; that's fine, it's great for people who don't want an overly complex email process. Keep it! But add a third option for power users that want to put

<img src="mysource" width="50%">

or

<p style="text-align:center;">MY PARAGRAPH</p>

into the email (hopefully that appears in plain text when I click post idea, otherwise it's gonna look weird). For me, the best thing I currently know of to compare to is HTML view on Blogger Articles and pages; there, when I am writing an article, I can switch to HTML view and easily correct weird bits of code or do other edits I otherwise wouldn't easily be able to do. I know Blogger/BlogSpot is not an email client, but it's the best demonstration I can think of to explain what I'm talking about.

The W3Schools website also will show a preview with their HTML editor simultaneously with the HTML code, so perhaps, that's one step farther for this feature idea.

I'm not sure how much of Proton's user base is made up of power users, but if Proton is trying to get more normies on the platform, then the edit HTML feature could be hidden behind an extra menu or something so that the normies don't accidently click into a POV they don't understand... maybe power users have to enable the button in settings to then have the third option... just an idea if this would be a problem for normies (I know it can feel weird to see something (like html) you've not really seen much before, so I sympathize with normies on this).

I think one other person sort of mentioned this, but I wanted to give my own thoughts on this feature idea.