Show if a incomming message was encrypted or send in plain-text
Show the difference between a message that has just been received via SSL and a message that was encrypted using your public key.
Currently there is no way how you can see if a message was encrypted using your PGP-Key, when send from the outside.
This already exists. If you hover over the lock next to the senders' address you can see a tooltip explaining how it was encrypted. Messages that arrives unencrypted and are encrypted with your public key (such as messages from a Gmail contact) will have a grey lock. Fully end-to-end encrypted messages (such as from another ProtonMail user, or other PGP enabled service) will have a purple lock.