We are a startup that just launched (https://tmail21.com) that aims to rethink the discussion itself. In our view, one of the major problems of e-mail (and chat for that matter) is that the discussions are not goal-oriented. So, discussions/threads can meander around with no outcome or accountability.
So, we enhance the notion of discussions to be goal-oriented.
Now, once one has the notion of goal-oriented discussions, a natural next step is to evolve a goal-oriented discussion into a Lean Process (which is basically a goal-oriented discussion with more structure) . Examples of a Lean Process might be a Blog Article process, a Product Deployment Process, a Feature Design Process, an Issue Escalation Process etc.
We've done all this in an email-LIKE interface. I guess we'll leave it to others to decide whether this constitutes innovation in the e-mail space :)