I seems like you're restricting yourself to graphical clients, ruling out the (minor) changes introduced by things such as notmuch, etc.

At the end of the day what people do with email is very much the same as it was 20 years ago:

* Send it.

* Read it.

On that basis there's little innovation that seems missing, unless you're thinking of something special?

Me? I like console mail clients, so I wrote one with built-in scripting via lua. It makes me happy, but it'll never take over the world:

http://lumail.org/

I suspect my next task is to try something graphical, but I'm in no rush.