A photographer run coop would be great and I'm always open to fair trade agencies and am happy to join the discussion. However ...

Expecting 100% commission and for those who do the work behind the scenes to do it for free, in my experience, is not the way to go. I was involved with a start up agency hosted by a fellow photographer who I met through Photoshelter (during its brief honeymoon as an actual stock photography agency). As here, there were a group of us who'd gotten to know each other through the forum that this photographer hosted. Most of us were new to stock, though he had been licensing work on his own for years and had some publishing contacts.

It was to be a niche agency and he set up a beta site with ITPC intake, bulk uploading, a forum and left it to us to self curate which by and large people did a good job at. The site was up (though private) for a couple of years starting in late 2009/early 2010 as about 100 of us (in retrospect too small a number but we were grounded in the traditional RM stock agency paradigm) dutifully uploaded our photos. He and his wife did all the backroom stuff paid for the software and upgrades and stayed active in the forums. We discussed pricing models, licenses, etc and agreed to a mix of RM and RF non-exclusive stock and to pricing. The forum was far far less contentious than here, but moderators were still necessary. I forget what the goal was in terms of numbers before he was going to start marketing the site, but by the time we got close to the target the market had changed dramatically. Though we had agreed on just about everything, he realized that he'd never be able to compete with what was out there, and the project folded. Angry people who'd spent a lot of time for a dream, but most understood that life sometimes gives you lemons. 

Had there been a dedicated paid team behind the scenes, and had we been paying while the site was in beta, we might have hit our numbers sooner and perhaps jumped on the bandwagon while there was still room. So, I'm sure you can understand why I agree with those who say there needs to be a commission/salary for those doing the coding, the marketing, the curating, trying to get investors, etc. You can have the best photos in the world but if no one sees them, they won't get licensed. You need to sign up photographers who will share their enthusiasm with others and help you market - that's why, for example, all the micro sites have referral programs, it just makes good business sense.

When I consider that I license photos daily on SS despite the fact that my 200 photos make up just 0.00057% of their database, it tells me that they know how to market my images. There are many times I think that I should have 1,000 images on SS and 200 on the traditional agencies and not the other way around, but I hate the fact that they charge so little for my work and that I get such a small percentage. I know Walmart makes more than Neiman-Marcus - guess I'd still rather be the later.

I think Stocksy has a real chance because its founders understand the business side of things, and I assume that most of their photographers are not naive amateurs but rather come from the traditional side of things and don't expect to get rich overnight. They also have Bruce's millions and I'd assume there are investors as well.

If people think spending $120 for updates (via the symbiostock discussions) is too much, however, then I don't think they really have a concept of what it takes to run a successful co-op. I've spent a portion of my time over the past couple of years contacting potential stock photo buyers and have been licensing photos directly, as well as through those who discover my work on my website. I do mailings, pay $500 a year for a Pro site on Photoshelter (one of the best investments I've made), do research, and have tens of thousands of images on my hard drives that I'm keeping track of. It's a long-term investment of my time, and the momentum is starting to build. It's not going to happen overnight. Now multiply that by 100 people or 1000 and think how much you need to  do and to spend to market their work. The cost of hosting and writing the software is a drop in the bucket. You need a marketing team, a legal team, a sales team, an advertising budget, and I don't know what else and sites that have all that and major investors as well still don't make it. 

Sorry to go on but just some of my thoughts on the matter.

I'm sure the OP knows a lot of this and has given it a lot of thought before sending up a trial balloon. He's looking to gauge interest. None of this is rocket science, but it is hard work, time and money. If it was easy no one would be selling their work for pennies and watching the site owners make millions.