It would be nice to be able to switch between plain-text and HTML composition modes in the email-composition window.
Today I had to change my account settings to send an HTML e-mail. This should be possible easier, even when the account default is to compose in plain-text mode, there apparently are cases when this is desired. I could imagine an plain-text/HTML button in the button bar with the "send, spelling, security, safe" buttons, or an option in the Options menu to compose HTML when the default is plain-text or vice versa.
I would be OK with a warning (or even, would like to have a warning) somewhere when the account-default is plain-text and the composition mode is set to HTML, since HTML e-mail is seen as unwanted to send in this case. Vice versa, I could imagine that people who normally send HTML mail would like an explicit notice that they are sending plain-text now.