I think it's high time to revise the IMAP/SMTP interface for clients anyway; the JMAP spec that fastmail has put together is pretty interesting: http://jmap.io/
A really good way to encourage innovation in the mail client space might be to write a proxy server for JMAP (or something similar) to a legacy IMAP/SMTP setup, and hide all the legacy IMAP/SMTP issues inside the proxy so that building a client anyone can use could actually be fun.
I'm planning to chat to people at OSCON later this month about JMAP, and we're making a big push at FastMail to do exactly what you suggest - build a proxy that can wrap any IMAP/SMTP setup.
It won't be quite so fast as running a server that can support JMAP natively (we also plan to build an open-source JMAP server on top of the Cyrus IMAPd that we use internally) - but it will bridge the gap :)
A really good way to encourage innovation in the mail client space might be to write a proxy server for JMAP (or something similar) to a legacy IMAP/SMTP setup, and hide all the legacy IMAP/SMTP issues inside the proxy so that building a client anyone can use could actually be fun.