I think this could be a marvelous way for the fanfiction writers who have huge followings and spend a lot of time on the writing to get some cash they deserve. If they keep publishing other fanfics for free, it wouldn't anger too many of their current readers either. I'm currently reading a fanfic with 141 chapters, and I really wish I could pay the author a small amount, even though they cleary need to edit it.
This way people would actually become professional fanfic writers, a mix between a fan and a professional author, something new all together. With the original creators permission and payment, it's a actual job.
The argument "fanfics should be free" sound a aweful lot like the argument "all entertainment should be free, and the creaters should be doing it for the honor".
Honor doesn't pay the bills. Think of the marvelous stories stuck in peoples or note pads heads because they can't justify spending so many hours writing for free. That's true for non-fanfiction writers as well. Writing is hard to make money from.
I doubt this will go anywhere though, too many restrictions and the really goof fanfiction writers would (and should) want to keep the rights for their plots for themselves. But it might be able to work, if enough people joined(and you could get the option of a subscription), and you would get to re-use your own ideas/plot/original characters and as a buyer you could sample fanfics.
But that's a lot of ifs.