As I have said many a time, there is not more room in the market for all encompassing imageboards. I believe that specialty and splinter imageboards are the only ones that have any hope.

Case in point is my operatorchan.org site.

It would have not succeeded if it was not a niche imageboard. I don't add any boards that don't directly relate to the niche my site is in. I also won't add boards that won't get almost any traffic unless there is good reason to. Most imageboards sites fail this way.