Ahhh.. I already had the same thought of using the memo key for encryption and decryption. However, this was previously attempted and there was a lot of spam which resulted in the increase in the log size of SteemD and was not something the community liked. There is something that steemchiller and I had a chat about called https://matrix.org/ This is a decentralized solution that would be great to have. Might give this a try sometime this year.