I think a first thing DC could do is a setting to prevent any plaintext messages to be sent, such that you can’t accidentally reply in the clear. DC could also have a way to “confirm” a PGP key after the chat has been created, such that the contact can gain the 
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The WKD/Autocrypt situation is larger than this and I took the liberty to reply in the other thread (The life-changing magic of WKD)