Third party spell check support
I am dyslexic and as such have settled on using Grammarly as a way to spell-check and audit all of my outgoing messages.
Even though there is a built-in dictionary in both the web app and mobile version (supported more by the dictionary of the keyboard on the app) I find them really lacking and I find myself roundtripping between a word processor and the actual interface for every email as I'm anxious the built-in filter won't catch most mistakes.
I'm aware of the privacy and security flaws of using such systems but a toggle to allow them would be great when I'm not dealing with sensitive info.
The alternative is spending even more time and clicks trying to do something that for most people is easy.