This. Very much this. I play on a relatively low-pop PvE server and I've never wanted more people around. I have all the people I want in my guild. That said, the CRZ idea is interesting, but so far it isn't solving the real problem - the sparsely populated Auction House.
Bring a half dozen realms together, but do it all the way - not just the players interacting in the environment, but the money and the market too.
*B*