I like randomness in fights, but not in the way that mechanics or their timings vary a lot between different tries of the fight. More like... randomly targeted players, random areas of the platform becoming unsafe, random kill order on mobs or mechanics pointing in a random direction. These types of mechanics force you to react during the mechanic or afterwards (all run out of the spawned fire), instead of preparing for them beforehand (place the fire in the assigned corner).
Examples: a persistent dot can jump to the next randomly selected player after it expires, a random player can be targeted for meteor placement to a randomly selected corner, puddles of fire may appear in random and sometimes inconvenient places throughout the fight, adds can be randomly targeted by a short debuff which makes them vulnerable, or 8 stationary adds around the arena can force you to quickly look away from them in a random order.