Instead of lockout timers have a scaling system where you can raid as often as you want but it will increase in difficulty.
Difficulty ramp will decay with time, so if you prefer laid back raiding you can wait the normal 3 days between, if youre impatient and have resources to burn you can risk going in again immediately.

To determine difficulty setting you could either poll each player and use the highest ramp point found (this must be visible atleast to group leader when inviting) or you could take the average of the whole group (this could help new players get in raid groups).

Ramp must be pretty steep ofcourse, with top point at 12 players who have just finished it being a real nightmare and challenge.

Could be a good addition not used in any other mmo that I know of aswell as a nice resource sink.