The difficulty on many group/world bosses is a fun challenge for the individual that isn't totally min maxed and smaller groups that aren't min maxed at all but, it isn't really a challenge for larger groups or min maxed players. I'd suggest giving bosses the potential to trigger Retribution Stacks to attempt to somewhat remedy this.
The Group/World Boss would have a damage counter that would calculate the non-Ultimate damage it took over an interval and if that damage passed one or more damage thresholds the boss would get a buff based on the threshold/thresholds passed for a longer interval that could stack. The buff would influence the boss's ability cooldowns, damage mitigation, and damage dealt to help it be more of a threat/less of a disappointment.
As the buff would only trigger if a damage threshold was hit, it would hopefully not mess up the experience too much for players that like the current difficulty as it would hopefully not occur much for them.
With the Zeal of Zennithar upcoming, it's likely we'll see groups of maxed out characters going from world boss to world boss and shredding them in a few seconds. Having something that would slow them down might make for a more enjoyable experience.
If the Retribution Stacks worked, it could potentially be refined to make certain quest bosses more challenging in some situations and be used in the reverse fashion to make older group content such as Harrowstorms or Dragons easier if the amount of damage incoming was below expectations for an extended period.