TL;DR: This is a well thought-out crowd control solution featuring resistance rather than denial. Please read the whole thing.
I’m sure many others will agree with me here; the current crowd control mechanics for bosses doesn’t work as well as it probably should.
For those who aren’t fully aware, currently bosses keep from being stunlocked by having an ability called Unshakable. When players apply a crowd control (CC) effect to a boss, that boss gains three or more stacks of Defiance. Each stack renders it completely immune to one CC application. Once all the stacks are gone, the boss is vulnerable to CC but instantly gains more stacks as soon as it is hit by one of these effects.
This system is problematic, especially for open world fights and LFG dungeon groups. If dungeon runners carefully group up and communicate thoroughly it typically isn’t a large problem, but it still makes CC feel ineffective overall. In open world fights, there can be dozens of people spamming crowd control constantly, so the instant a boss loses all Defiance stacks it usually gets hit and gains tons of Defiance again. This can result in ineffective use of CC, as the boss is extremely likely to be immune to interrupt at key moments.
Instead of complete immunity from the start, I suggest that Defiance stacks become a diminishing returns effect that can fall off over time. These stacks should build quickly to still be a challenge in small dungeon groups, but have a cap (let’s say 25) so that the boss can’t be completely immune when under fire from a large open world pack. Each stack should also fall off after some time (let’s say 45 seconds, for an off-the-top-of-my-head example). For every stack of Defiance, crowd control effects on the boss should have their duration reduced to a point (example: a 3 second stun might become a .25 second stun at 25 stacks), as well as the boss becoming immune to CC for a few seconds after the effect ends (let’s say 1 to 10 seconds over the course of 25 stacks).
For bonus points, each type of crowd control could receive its own Defiance; this would prevent Slows from making the boss immune to a stun at a key moment, for example.
With this new system, you’d still have bosses heavily resistant to crowd control against mass groups of players without total immunity. Players could feel like their CC means something rather than just being a thing to get rid of stacks. In dungeons, if Defiance stacks decently quickly, players could CC the boss at key moments to interrupt massive hits and dangerous attacks, and carefully wait for stacks to fall off before coordinating their attack to (very briefly, because these stacks build fast) lock down the boss and burn their health. It also creates an interesting set of choices for dungeon groups, who would now have to decide between saving their CC for a burn or using it to keep from taking too much damage.
I really think such a system would take a good bit of effort to get the balance right, but overall would be a huge improvement, and would make PvE content much more fun for everybody involved.