I would like to see an "Undo Send" feature, as found in Gmail. This is essentially a delay in sending, I guess, during which you can click an "Undo" link that appears in the bottom of the screen after you have hit "Send" on an email, for a varying amount of time that you can set (ex. 20 seconds). This gives you the chance to abort the send and tweak the message if, after hitting send, you realized you had forgotten something or you wanted to change the tone. It's something I find I use quite often for work.
Totally for this idea! As a business user, there are times when you have to redo the content of the email before sending. Every word is crucial so you really have to think about what you are saying and this Gmail feature saves a lot. Would like to see this in Protonmail because I'm leaning more and more to using the platform for my businesses, but the features are still lacking.