In an ideal World, you would allow everyone to join what map they choose BUT before you could enter a map it would look at relative populations of each server ON ALL MAPS NOT JUST ONE. If the total number was too high for your server, you would be put into the new “just for fun but not for points” map. It wouldn’t have to be equal but it couldn’t be out of hand. That would also reduce the silliness of servers buying foreign guilds for off hours coverage. Ok, but if there is no competition, everyone will be on the overflow maps anyway!
You still get to WvW but generally speaking the competition maps would be more or less balanced because of the overflow map.
Bored of the overflow map? Well there should be free transfers to underpopulated realms.