Maybe a stupid idéa.
But why not have an option to setup the server, to connect to a complete different server when queue and entering dungeons and battlegrounds? (And also, be able to configure the server to act as a dungeon/bg-server only.)
I thought it becouse maybe the resources is less used at a computer when having a server that is only for dungeons and BG's, instead of running the whole world + the bg's and dungeons. Then a lot more players could connect at the same time, at a server like that.
Players at a normal private server with low amount of players, can connect to these dungeon/battleground-servers to meet more people in instances.
I don't know the problems that can be connected to this idéa though lol.