Basically, you're more anonymous the more people use Tor, so increasing Tor's usability increases it's security.
Another thing is key generation. Encryption people demand perfect randomness for the key generation, and that means the key is this crucial piece of data that must be perfectly protected and copied about. That's unusable for most people. Instead, pick the key from 1 billion derived from a user's password. If they use a different computer, or reinstall their OS, or whatever reason then the software just takes a while to try each billion until it finds the one that works.
Or encrypted email. For perfect security you need a safe way to exchange keys beforehand and all kinds of trouble. No. Just have software attach your public key in unencrypted email to recipients. If you receive an email with somebody else's public key then the software starts encrypting to them using it. It's insecure in so many ways, but it would mean the majority of email being encrypted and if done right with almost no impact to the user (password-derived keys, password change automatically sending yourself a new-key-encrypted email with the old key, etc).
Security people need to stop seeing things in black and white. Something can be insecure and "broken", yet still raise the overall level security.
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.
If you build a "perfectly secure" piece of software, but it takes a very high level of skill to use it, your users will use something else that is easier to use, but less secure. And then how has your ideologically perfect piece of software helped improve their security?
If you make tradeoffs for usability, you will raise the bar because people will actually use what you make.