From the beta testing we’ve seen, it seems that the only additional mechanic is the two new types of adds to deal with after crossing through the barrier. There was also a new kind of laser in the final room, but it was supposedly removed after testing.
The two new types of adds are spawned when either a tank or healer crosses through the barrier. When a tank crosses, it spawns a bigger add that buffs other adds and needs to be tanked (item). When a healer crosses, it spawns an untankable red orb, which repeatedly puts a debuff on players that needs to be dispelled when each player is 10 yards away from everyone else (item).
The new laser that may have been removed was a big cross on the ground that rotated around quickly, and it was only in the final room. It was simple enough to avoid, but combining it with other mechanics was a nightmare.
There may be more changes to the fight not shown in beta testing, so we’ll release a full Mythic guide after all changes have been confirmed.