Tip #1: Don't Panic - WoD is probably 6 months away or more. Panicking now is silly and unproductive. Focus on what you were focusing on before the big announcement, your next progression fight.
Tip #2: Be patient. No serious raider will want to join your guild now for a bench spot or a flex-only raid. Some people I know are starting a 2nd raid team. I'm not a fan of this idea because I think it will split your focus. Just wait it out, Blizzard may or may not change their minds anyway (I hope they don't, and this is from a current 10 raider). I think people should be waiting until closer to WoD's release to worry about seriously expanding their rosters. But if you don't want to wait, only recruit up to 11 or 12 so you can rotate them in with your regular raid team as needed. This will reduce your mythic need to only 8 or 9 more.
Tip #3: After WoD's release is the time to really start expanding. You can start running Normals and Heroics right away with anywhere from 10 to 25 people. Recruit slowly. Yes, you are going to lose the initial progression race to the current top 25 person guilds who don't have to worry as much. Focus on what you want to accomplish, not what others are doing. Unless you are at the top 5 guilds your ranking really doesn't matter a whole lot. If you have trouble recruiting the kind of people you want, you will be able to draw on your past guild successes. Everyone's going to be in the same boat. By the time you finish Heroic Progression you can get to at least 20, and if you're not there yet, well, you'll just have to rely on Tip #2.
Tip #4: You will have more recruiting tools after WoD is released. The in game raid browser will be much improved and you will also have the ability of pulling in people from other realms for both Normals and Heroic content. This way, you can get to know them before they join your raid full time.
Tip #5: Even if you can't fill 20 spots by the time your guild has cleared the heroic content to unlock Mythic level, because of Connected Realms there is more than likely going to be many fine, well geared people from other guilds who would love to pug into a Mythic 20 raid where most are from the same guild, so you will be able to start on the initial Mythic content even without finishing your roster expansion.
Tip #6: Make sure your current raid team knows what your plans are so they feel connected and can follow tip #1.