Enhance Composing functions
There should be a few more composing features such as 1) more robust font selection; 2) indent feature; 3) spell check feature. These are things that are available in any basic email program. Also, the ability to color/highlight text, embed images, and emoticons would be desirable as well.
Great Afoofum commented
If one sends email using HTML format, then sure, these are definitely necessary. However, they're pointless if one's mail is encrypted with PGP, because what one gets on receiving such a mail is the HTML source (which is not as easy to parse as plain-text or markdown, even for a Web developer).
If you're not using ProtonMail with PGP, why bother?
Personally, I'd like to have the option to choose to send and receive *all* my mail as plain text (with HTML format mails converted to Multimarkdown or Markdown Extra on arrival). I can't find that feature in the settings and suspect it has been removed.
To add to this list, I would like the ability to increase quote level multiple times. The new Beta ProtonMail only has the ability to add or remove the single quote level.
And can we have more text colors (or maybe a hex entry) and the ability to select an in-between font size?
More robust font selections should be considered as this is something every basic e-mail program has. Spell check feature is a necessity as well.