UI change request(s). Say you are in the messages tab, can you change the "messages" tab to then say "Refresh" so if it is clicked again it will refresh. Seems silly to have to hold it down for 2 seconds when a double tap can do the same thing.
sending a message window. At the top right can you add a box with a "+" so that it is easier to add another contact to the conversation (or a person icon will do ). Switching back and forth to the main DC window is not overly difficult, but can be made easier. I think making the SMS and dial buttons larger can also pay off