I'd like to see an option to upload a public GPG key to the forum.
After that, user would be able to enable other users to send encrypted PMs. Or encryption can be enforced.
If encryption is enabled, 'Preview' would be either disabled or run completely offline on javascript.
When the message is sent, it would be first encrypted offline using
openpgpjs or similar.
Decryption would be optional. Probably it'd be better to keep it off the site, so no private keys are uploaded.
For extra security, there can be information regarding when a public key was uploaded, in case a hacker accesses an account an uploads their own key.