I think it would be hard to narrow it down further - the problem has more to do with the fact that hackers don't want to bother with marketing and/or negotiating deals, myself included. I mean, the markets that would work are anything large enough to have several startups operating within it without being in competition with each other, but like I said - the problem is a bit more complicated than that. (I'm working on a book that is somewhat related to all of this, but I keep getting distracted with making things.)

If you really wanted it to work: In my opinion the best way for something like this to gain traction would be to start small and make it exclusive. Make yourself the gatekeeper to a startup marketing nirvana rather than driving a race to the bottom. Play matchmaker with hot startups (taking a commission of course) - perhaps one of the niche app hosts want to pair up with a popular new domain name generator, or maybe you can find the next set of games to sell as a bundle. It sounds like a lot of work (and I'm sure that it would be) but the fact is that startups already have too many ways to find each other - a much more valuable proposition is someone willing to do the legwork. If you are able to generate enough interest in what you're doing, automating it and making it public might become an option in the future.

tl;dr I don't think hackers understand the value of what you're suggesting, even though I think it's valuable. Create demand and prove your method first.