The team at Blizzard have announced an exciting new flexible raid system, allowing any group size between 10 and 25 players for patch 5.4:
... we’re in the process of developing a new Flexible Raid system, which includes a new difficulty that sits between Raid Finder and Normal difficulty, while still allowing friends, family, or pick-up groups to play together. This difficulty will be available for premade groups of 10–25 players, including any number in between. That means whether you have 11, 14, or 23 friends available for a Raid, they’ll all be able to participate.
The Flexible Raid system is designed so that the challenge level will scale depending on how many players you have in the Raid. So if you switch between 14 players one week and 22 the next, the difficulty will adjust automatically. Keep in mind that unlike Raid Finder, no matchmaking is available, so you’ll need to make sure you invite people to attend—but if some can’t make it, it’s not the end of the world (or the Raid). You’ll also still be able to invite Real ID or Battle.net friends cross-realm. Who you choose to bring and what Item Level gear they’ll need to join your merry band is up to you, too—there’s no Item Level requirement for this Raid difficulty.
This new system will use the LFR loot roll mechanism and will drop loot of higher ilevel than LFR.See the full official post