For my use case, I would be happy with rotating gateway servers plus some intelligence at the primary server.
I like the idea of monitoring against blacklists, but it's actually more than that - the feature should ideally monitor for delivery delay responses so it can forward on to an alternate gateway.
My system primarily sends transactional emails, not marketing spam, but customers will type their email addresses incorrectly and when messages are delivered to the wrong people, users of those services (yahoo, gmail, etc) are trigger-happy and will mark the message as junk. The side effect for me is that the entire IP gets blocked or throttled, which leads me to a days-long effort to get the email admins at whatever service to respond to me (Yahoo just took one week to unblock one of my gateways) and fix their problem (while my clients are yelling at me).
Today, SmarterMail's outbound gateway mode transmits the message to an outbound gateway - and leaves it there. If it encounters a problem there, it's going to be stuck there forever (at least until the retries expire and the message bounces).
Instead, SmarterMail's primary server could send the message to the gateway, which could try to send it, but if it encounters a delay or other such block, it could pass it back to the primary server, which could then move on to an alternate gateway.
Then, if an IP gets blocked, I could temporarily remove it from the pool of gateways.
This would be a dramatic improvement over the current situation.