Back in Warlords of Draenor we got new lockouts, were we were allowed to kill bosses multiple times, but only get loot on your first kill. I really like that. And especially because we have no LFR, i think it would be a really good fit for MoP classic.
While you still get the same amount of loot, you could raid again on your off spec or you could join a pug, between guild raids, to practice, without screwing up your ID.
It would also be cool for people who only do pugs. Lets say you kill 5 out of 6 Bosses early in the week. With the current system you need to wait until you find enough people who are willing to join your group for the last kill only. With the Flex lockout you could join a group which is still on elegon, kill elegon again, without getting loot and then you kill will of the emperor with them.