WoW's entire Migration service was introduced to allow players of specific Factions to transfer servers. For 2 whole years it operated on this principle. I can't speak for it today, as its very much in the 'end-days' stage and they're probably open to any sort of paid-Migration transactions, but through the hey-day of WoW they had a monthly and limited rotation of server Migrations. You could only go from Server X (Horde) to Server Y (Horde), on account of server X having a Horde-Alliance ratio that was imbalanced, and server Y having a massive lack of Horde players, and so on.
The server migration never worked, the handful of servers crammed with players stayed crammed with players - just every once in a while a couple of guilds would transfer. The servers still had zero faction balancing, the vast majority of servers were still suffering from poor population levels. You can also still roll on any of these 'full' servers to.
Again, 'Full' does not mean 'Closed to players' in MMOs - Devs merely try to guide populations through various means they don't demands where players choose to play. It's why you get recommended servers, not mandatory
