It would need to be "insidious" than that, in a way. But yes, either of these mechanics, or combinations thereof, would massively increase the need for overall healing, to the point where DPS rotations and all would be far less of a problem:

* On each autoattack, the boss fires 2 shots on random non-tank raidmembers that deal 25%-40% HP in damage. Not that there is no lockout so someone can indeed be hit twice in a row. Heal up quickly~
* Constant DoT for a non-trivial amount, requiring active healing every 6-8 seconds (so 12-16 for each healer).
* Every few seconds, the boss fires a random non-telegraphed low-damage line AoE into the room.
* Every few seconds, the boss has a random non-telegraphed area explode for damage.
* All melee attacks by the boss also deal 40%-50% of a non-tanks damage to all other targets in melee reach.
* Boss reflects a portion of all damage dealt to it (not a lot, but enough to need constant healing).