For the love of all that is holy – please do not make this a FFA transfer.
Free transfer of Horde will pour into already Horde-heavy servers.
Free transfer of Alliance will pour into already Alliance-heavy servers.
FACTION LOCK servers who have a high percentage of one side already. This is such a simple solution.
Destination server has 60% horde already? Easy, no more Horde transfers. Same with Alliance heavy servers.
Just do it right, Blizzard. Please.