hmmmm i would guess a few things:
every 20% forces Anduin comes and you have to stave him off until he reaches 75-50% hp before sodding off, and we get a buff similar to the pride buff
random person gets MC'd every 5 minutes for 30 seconds, you can choose to CC or bring them to 5% (200-300% extra hp/damage) makes the pace of the pulls something to be considered.
a "tower sentinal" is placed in 5 locations about the instances that reduce secondary stats by 10%, can choose to kill or leave them (like with season 4 BFA)