I'd like to see mail clients disappear from production ... or if people insist on making them make them simple ... build one that does what chrome first did as a browser.
Make it get out of the way. We do not want to see it we want it to work. Plain and simple no "extra functionality" that we did not ask for. have it simply interface with things we already use, whether that be services or websites and have it let us get back to our work faster.
As it stands email is what drags productivity down in a lot of offices and that is not what it was meant for, integrate your system where we can read and respond no matter what computer we're on or operating system we're running quickly and then get out of our way so we can go back to our tasks.
maybe features that will alert us to emails from certain users with a pop up while not alerting us of others where we know the mail will be less important.
just a few thoughts, there is tons more I could say here but I'm sure most people could care less what I think. hope that helps with your research