Couldn't you just give those conversations an Unreplied label? It will show up on the left.<p>Gmail's labeling system is probably the most useful but overlooked feature they have.
You can just build an extension or addon. I don't see revenue for that kind of model (the only possibility is that someone makes a dashboard aggregates data from multiple accounts, multiple providers). You can try to achieve this with the help from reusing <a href="https://github.com/KartikTalwar/gmail.js" rel="nofollow">https://github.com/KartikTalwar/gmail.js</a> or similar attempt.
Building a platform on top of a Google product, specifically Gmail, is a recipe for disaster. I sincerely hope that nobody who cares about their money or sanity does so.
I want the opposite.<p>When I receive a mail, I want to set a timer saying: "this needs to be answered before noon", or "today" or "by the end of the week". Then get relevant reminders.<p>That way I can go back to whatever I was doing and have it out of my mind. For now I just star messages that need to be replied to, but that's too coarse-grained.
Check out ActiveInbox: <a href="http://activeinboxhq.com" rel="nofollow">http://activeinboxhq.com</a>. Been one of their happy customers for a long time now. When reviewing an email, I hit 'waiting' on their action bar, and then they show up as a radar list (their take on multiple inboxes). Loved it so much I paid for it.
In my email, I have "Waiting for reply" and "To reply" labels (besides a few others). An algorithmic solution to auto-apply the labels would be nice (currently all new outgoing emails are labelled as "Waiting for reply").
Co-founder of Streak here: we're happy to add this if there is enough demand. Just email us at waitingforreply@streak.com. If we get enough interest we'll add it tonight. You'll be able to use it for free.
I have written a "snooze this email for X days" extension for gmail. After X days the email is marked undead and moved to my inbox.<p>The extension is server-side, so you're not tied to your desktop browser.