I like this idea, which I came across today when looking to see if there was any discussion here already about another issue I came across when using the flag → send user a message feature. Dropping it in here in case we decide to give this feature some love.
This is minor, but I found myself wanting to add a friendly emoji but it was not as easy as it is in the composer. They work, but you have to know the markdown or go back to the message later and edit it. There is no emoji suggestion pulldown as in the composer normally. Same thing for @ or # to search for a username or category/tag/chat to include in my message.
Edit: ah, following the related topics I came across this other related topic, which covers the issue I brought up and more. Phase out redundant flag PM in favor of full PM functionality