I have been using Disa exclusively as my messaging app since it was first released. It's great! A few ideas though..besides the obvious integration of GV, Hangouts, FB, etc..
-separate ringtones for each service
-quick reply feature in notifications with preview of text and screen on option or popup and screen on with incoming message.
-option in SMS to see when a message has been received and/or ability to see when someone is typing (not sure if this is possible, maybe disa exclusive?)
-ability to select ringtone notification
-import current contact pictures as thumbnails in conversations
I'm sure I will think of some more later. Great work thus far!