That's a broken financial model. The intersection of people with the capabilities, ideas, enthusiasm, and available time is extremely small. Actually, the highly skilled people are least likely to be available because they are most likely to be working already. My apparently crazy idea is that we need better financial models first. My favorite pipe dream is a kind of a crowd-funding model around clear project proposals.
No, ideas are a dime a dozen. That's the delusion most of these proposals have, that if only they got to share their great proposal with the world lots of people would come help pay for it and lots of developers would come do it for little or nothing. Your proposal sounds extremely similar to other crowdfunding / bounty / donation proposals that have been done, but most of them amount to "Now I've made a tip jar and put in the first $5, why is nothing happening?"
If you're real lucky you find a project where you put in a feature request and somebody says that's a great idea, I'll do it. If you're hiring at full commercial cost, there's tons of contractors willing to do it. Between there you might find people willing to work on it for everything from beer money to paying the bills, but then they mostly work on what they want, not what you want because they're contributing most of the value. The good thing is that they're usually in control of the scope and complexity of the tasks they agree to, so you usually get what you pay for. Still due to whiny brats it's best to put up a tip jar with no guarantees.
If you're looking for someone to create something that doesn't exist and thus probably is nobody's itch, you probably have to get close to commercial funding. Maybe some will do it for somewhat less since it's non-profit and for open source, but not beer money cheap. That means you have to get lots of people on board, which means mediating between all their pet ideas. And when push comes to shove you have to actually have to both get the funding and find someone willing to do it.
What you describe is the perfect waterfall spec, everything is described up front down to the smallest detail. Everyone who's worked with it in the real world knows it's a giant pain in the ass to create, which is why they go agile. Most likely it will have flaws and then the fun starts dealing with your co-sponsors and developer complaining about any inaccuracy in the spec, delay in delivery and what actually constitutes fulfillment. And you don't have any budget or power to approve change orders. Worst case you have a lawyer on your ass because the developer is fed up and wants to get paid.
...at which point 99.99% of the people with ideas will have said "shit I didn't want all this crap, I just had this great idea.... you fix it" and disappear in a puff of righteous indignation that the world didn't just take their great idea and ran with it. I mean that was the hard part right, like coming up with the script for a movie. Once you have that, actors, directors, producers and camera men will come running... or maybe not. I think you can build any platform you want for script writers and movie producers to meet each other, but it won't change the fundamentals. Same with idea people and open source developers.