I currently use Mattermost from the web client and an iPhone. It would be great if draft messages were synchronised between the different clients.

This is not something that sounds particularly useful in the abstract, but it is a feature of Telegram and I found I would often write half a message and then remember that I had a relevant link or similar on a different platform, so want to continue the message where I could copy-paste that link. Now that we have switched to using Mattermost, I am missing it.