I can think of things like
1. built-in mail merge and updatable contacts where someone can directly reply to your mail-merged email and unsubscribe
2. shareable urls of emails - I hate that I can only forward/cc people to share an email with people. Shareable emails will make bookmarking important emails in the browser MUCH easier
3. sidebar comments/discussions section built into the email inbox instead of unending chains of reply/reply-alls.
Anything else that could make the email inbox more interesting?
The only thing email needs is some progress in spam filtering like needness to do a microtransaction for opening an ability to write me. It will make an easier life for new mail providers and if I really want some spam guy to send me 10 letters a day I would pay him a dollar for letting him to send me 1000 letters.