Im sure Blizzard and players alike agree that the pool of available mechanics is going down the road towards staleness. They arent exactly stale just yet, but with some wild perspectives WoWs raiding could reach that point much later rather than sooner.
My idea: Time Travel phase. Similar to Garaj'hal with people choosing to be ported. Part of the raid gets sent back in time in the same room and encounters other mobs and objects that they need to interact with. A giant mob could be killed which unlocks an object to interact with. Interacting with the object can change an aspect of the encounter during the 'present' phase such as disabling an aoe dmg tick or activating an elite mode.
SWTOR had a couple of puzzle bosses that were pretty cool. It would be nice to see Blizzard implement something similar. It also had huttball which was one of the greatest pvp mechanics ever invented...I would gladly club a zooful of baby seals to get this type of bg implemented.