This cost money! but if you're one of those people that has a guild with 1000 members this may be for you.
Step 1: Get a level 25 guild with a ton of active members (I'm looking at you Horde on Illidan or Alliance on Sargeras)
Step 2: Transfer guild to another high pop server or to your buyers server (Tons of threads on official forums of people WTS & WTB guilds)
Step 3: Let the leveling commence
The guild will be turned into a level 1 after you transfer the guild, but you'll keep all your members. If you have a active guild, people will start leveling the guild super fast and you'll have another level 25 guild in no time. I've done this twice and while it may seem costly if you find the right buyer and transfer to their server you can make nearly 100-200Kg per guild or some cash back.