Things I wish messaging apps had:

1. Forward a message to someone else. Sometimes you're not the best person to answer, but a colleague is. Forwarding should be easy and feature minimal friction.

2. Auto-deleting of messages in a DM after X _messages_ (e.g. only the last 100 messages are retained as scrollback). It forces you to document knowledge in more suitable forms; than having it lost in DM silos. Furthermore, it keeps conversations with your regular contacts more candid and natural; but retains the information and context for infrequent contacts.

3. No typing status (thanks Cardinal!)