I would do something to entice guilds and players to move back to low pop realms. This will help with the low pop issues as well as help decrease the load issue with high pop realms (login queue times).
One thing they can do is grant perks to guilds of low population faction / realms. Perks like:
- Decrease lockout times on raids that have already had the realm first
- Extra loot drops from random bosses
These perks will last until the faction is to a set population for a set period of time. There would need to be things set up to prevent abuse of the system. Like maybe a increase cool-down on transferring out of the realm once transferred on. A cool-down of a set time after the population reaches the limit before you can relocate.
Also have the populations faction based to help balance the factions as well. So on any given realm the Horde may meet the population to not get perks anymore, but Alliance may still be extremely low and will still receive them and vise verse.
Not only have this for the guild but also player level as well. Maybe a "lucky" perk that does something similar to Fiona's Lucky Charm. Things that will help and encourage game play, but not something like a mount or pet, etc.
Also a discount for multiple character transfers to a low pop realm. So instead of $75 for 3 you can pay $50 for 3 (buy 2 get one free). Maybe even waive the faction change if you are transferring to the lower pop faction on the realm.
I am against these perks being something like a mount or pets, etc. Unless it is something lost upon transferring off of the realm and is only accessible to character on that realm (not account-wide). Could cause people to rethink transferring off if they loose those.