Maybe I'm not saying anything new here, but I would like to point at this post I just did about StackExchange website proposals. I'm ready to copy it here if you wish, but it's particularly long. I accept proposals.
The main point I want to make is that, without a plausible coordination effort, some sites will never have a chance to arise, even if there would be momentum. High fragmentation (and assuming disjoint communities, of course) lead to very specific sites that don't stand a chance of getting the thresholds, while a more general SE would. Unfortunately, this requires a self-coordination that I don't feel like it's coming, due to personal stance, I assume (nobody wants to give a bit), and the factual point that it's hard to coordinate such effort through the site itself. "Follow" requests spread out, and the result is that no sites will get to open.
I think the most strong example of this case is musical performance. A SE on such argument has to open, I simply don't believe momentum does not exist. My contribute to such site would be very occasional, but my point is on observation that the structure seems to be deadlocked on "Guitars" and "Audio Recording and Production", the first highly specialized for a specific instrument, the second more towards post production and sound engineers than actual musicians. For every other instrument, the situation is not moving forward, but I guess that coalescing all musical performance (from theory to post-production) into a single Musicians SE, it would open fairly quickly.
I would like to hear your opinion on this regard.