admirable to want to tackle a big problem, and 'make your own dream job'.
'if email actually worked'.
"worked" for whom? The fundamental problem as I see it with approaches like this is that email is a multiparty system, and it won't "work" for your life because other people don't share your views of how email should "work".
"For example, send all messages you've been CC'd in to be reviewed when you have more time"
You're assuming that someone who cc'ed me on something doesn't want a response right now.
"automatically have responses to a job opening put into a specific review list"
Assuming people email in with appropriate subject headings or specific text.
There's as many different ways people use email as there are people just about, and email is almost always a two-or-more-way street. Modifying how I consume and use email on my end can be helpful, but only to the extent that it doesn't violate the process on the other end.
I didn't see anything about an API in there. Having an API to program against would be useful, and an API that would be able to work against the multiple aggregated providers would be really useful. Scriptable incoming email processing (not necessarily realtime, but close) that could move your mail around and integrate with other services would be really useful too.
"Email requires action"
If that's your fundamental premise, I can see how that would shape your thinking and design. It might not actually be wrong, but I'm not sure it's necessarily right, or at least right for a lot of people. Unless you count "reading" and "delete" as actions too.
Subscription software - not necessarily against that, but I'm presuming this will be 'on the web' hosted? Not local to my desktop? Hrm...
Something I'd like to see is good search and ability to search through related messages while I'm reading a specific message already. This is broken in every mail client I've tried. I can read, or I can search, but not both (without multiple tabs in web-based mail clients). Reading a message and not being able to get more context and info from previous conversations is frustrating.
Good luck!