Problem:
PGP is complicated (VERY complicated, to the average user), resulting in next to zero adoption.
Suggestion:
Simplify the goals in a way that can be upgraded at at some later date.
I think we need a browser plugin (All browsers. Other non-browser tools too, ideally, but the browser is important) that lets you securely SIGN posts locally in a style more or less like GPG's --clearsign option. Ideally, this should literally be --clearsign for compatibility, with the plugin hiding the "---- BEGIN PGP SIGNED MESSAGE ----" headers/footers, though these details are less important.
The key should be automagically generated, and stored locally in a secure way. (Bonus points for leting you use the keyrings in ~/.gnupg/ as an advanced, optional feature). The UI goal is to simply let people post things and click a sign this button next to a or similar. Ideally, later on, this could become sign-by-default.
On the other side, the browser plugin should notice signed blocks of text and authenticate them. Pubkeys are saved locally (key pinning). What this provides is 1) verification that posts are actually by the same author, and 2) it proves that someone is the same author cross-domain (or as different accounts/usernames).
No attempt is made to tie the key to some external identity (though this would be somewhat easy for to prove). The idea is to remove the authentication problem (keyservers/pki) entirely. This can be man-in-the-middled, but the MitM would have to be working 100% of the time or the change in key will be noticed.
No attempt is made regarding encryption (hiding the message). This should also greatly simplify the interface.
The goal here is to get people using proper (LOCAL STORE ONLY) public/private keys. The UI should be little more than a [sign this] button that handles everything, and a icon on the reading side. It should be possible to get the average user to understand and use such a tool.
Later, when the idea of signing your posts has become more widespread and many people have a valid public/private key pair already in use, other features can be added back in. As those "2nd generation" tools have a large pool of keys to draw from, it should be easier to start some variant of Web Of Trust. Even if that never happens, getting signing widespread is useful on its own.
I realize this doesn't protect against a large number of well-known attacks, and only offers mild protection against MitM. This is intentional, as the goal is getting people to actually use some minimal subset of PGP/GPG-like tools, possibly as an educational exercise. The rest of the stuff can be addressed later.