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.
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.
rainloop[1] already does this.
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.