Fork Thunderbird or some such client, also make a web client available. Make a new service which offers only encrypted e-mail by default (with a new e-mail address that includes e-mail hosting for your own domain) and provides the key server and everything else. Advertise it as something different from e-mail like encrypted e-mail. Set a new precedent, create a new industry.
Most "users" don't care about end-to-end encryption and instant messaging has already become the "better e-mail" on phones. The only way I see of breaking into this space is to make something for companies, because they still have reasons for a "better e-mail" and value encryption.
But who would use it? This study is all about getting the average computer user to use PGP. Most people with webmail accounts won't want to switch back to a desktop client and possibly have to change their email address in order to send & receive secure mail.
Besides which, 'make a new client' doesn't answer the main issue, which is how to write a usable client. There are plenty of existing unpleasant PGP clients out there, unless you can detail what makes your new attempt better, it will most likely fail as well.
It has to be an all-in-one solution that you download that has to pretty much do everything for you so your mother can download a file and just go. With a simple wizard that lets you register a new e-mail address and potentially allows you to invite other users via their e-mail addresses... imported from Gmail or something like that.
I would not worry too much about compatibility with existing mail solutions or what not. It's a new service which is completely different from e-mail as far as the user is concerned. More advanced users can use it as e-mail if they want. Who would use it? Anyone who does not want all their mail to be a public spectacle.
Plenty of paranoid Americans out there right now that might be willing to give it a try.
Tutanota (https://tutanota.com/) does that, and even has compatibility with the existing SMTP network. It seems to be web-only at the moment though.
I'm pretty sure other systems exist, unfortunately as long as we stay with SMTP nothing will change.
No you need to provide a desktop client, open source, by default with everything configured in addition to the service. I guess you should also let users use other IMAP servers but the client should always encrypt all mails it sends out.
Hushmail was always open to court order attack, a desktop client, an open source one, is much less so.
It should always put the name of the client in the subject line or send some unencrypted text along with each message on where they can get a client to view the message. And the client should be able to configure itself with minimal input, sort of like Thunderbird does right now with most e-mail services. And generate, transmit, and store your public key to anyone upon request.
this is actually harder to solve than just 'here have thunderbird'. how do you replicate keys across computers? how do you send e-mail from a new computer when you don't have access to your old computer (it turns out the answer is you don't). how do you back up your keys? where do you store them?
there are some high security answers like "ship our users HSMs" but those are very brittle to common failure modes like "i lost it". I think you'll discover that if you try to create something that is both resilient to average negligent use but still encrypted, you'll wind up with something that is basically hushmail.
