I think there should be an option to be able to encrypt and send secure messages. Maybe make it an option when you click the 3 dots on the top right of the screens dropdown menu. And I believe these kinds of messages should be separate from the normal messaging kind of like how Facebook has its "Secret Conversations" feature. Finally, I think these messages should also be protected from others viewing them by locking all these kinds of messages with a password authenticator. (Password, Passcode, Fingerprint, ect.) Maybe make it a separate purchase after pro, people would buy it.
Essentially it's a Wickr or Secret Messaging feature.