It is with this same accessibility in mind that Blizzard has just announced Flexible Raids, a new form of raid content that will debut with the game's upcoming 5.4 patch. In the simplest terms, these raids automatically adjust their difficulty based on how many players are present, supporting any number between 10 and 25. The difficulty resets week-to-week; if 22 people come one week and 13 the next, the new raiders won't be locked into last week's numbers.
A new tier of item that falls between the Raid Finder and Normal difficulties is being introduced for Flexible Raiders, and Flexible Raid lockouts will be separate from those of Normal and Heroic attempts. Blizzard has also promised additional rewards for those who participate in Flexible, Normal, and Heroic raids -- rewards that will be unavailable to Raid Finder groups.
Flexible Raiding will be made available piece by piece as Blizzard tweaks the formula. It should be launching on the test realm Soon™.