So what do you think will make this this proposal a success when the two that preceded it failed?
I'm hard pressed to think of anything that would.
Doers, not abstainers
Consider for a minute, that we have a site for cooks, but not a site for vegetarians (though a couple have been proposed, and a third appears to be adopting the same "expand the scope until it's heavy enough to fly" tactic this one is).
There's also a site for brewers, but not for teetotalers.
Stack Exchange sites work best when they're centered around folks doing things... Not abstaining from things. If you're looking to cook or mix drinks without the use of meat or alcohol, you'll find expertise in those areas - but if you're looking for help sitting in a bar without a drink or a burger in your hand, you need a sponsor not a Q&A site.
A site able to answer questions on doing X as an atheist would be a site dedicated to doing X. Whether that's parenting, traveling, campaigning, or... cooking.
Of course, there's another aspect to all of these disciplines beyond doing and abstaining: proselytizing. They didn't call it a temperance movement because they sat at home drinking water, after all. At some point, you stop looking at yourself, and start thinking about your neighbor. And boy, does that guy have problems. If he stopped eating red meat and started believing what you do, he'd be waaay better off. If only you could convince him of that...
When two people share a goal - when they're working together to get something done - it's possible to formulate a clear, specific, objective argument for why that should be done a certain way in order to achieve it. But the broader that goal, the less clearly-defined the objectives, and the fewer assumptions shared between both parties, the more difficult that becomes. Pretty soon, you're left with the choice to either engage in perpetual debate, or just lay out all the arguments and call it a day.
We don't host infinite arguments here. If you actually desire a better understanding of the underlying concepts, then you want a site dedicated to examining those concepts. This has led to the creation of sites dedicated to food, drink, religions, and philosophy - if you truly want to understand any of these, even if you don't intend to adopt them yourself, then you'll find folks willing to help you.
What will make this third Atheism proposal succeed after the previous two failed?
If you want a Stack Exchange site, you'll pick a field of expertise, identify specific problems to be solved in that field, and craft a proposal that captures the essence of that topic and its audience. So far, I've seen no evidence that a cohesive body of knowledge or set of problems exists for any of these proposals. So, for now at least, the answer to that question is nothing.