Guild Wars 2

Invulnerability:
Until we hit 25 stacks of combo attacks.
Need to chain opposite type of combo attack.

Boss debuff:
After 20 secs of poison, boss becomes highly vulnerable for a short while.
Make a huge notification when defiant stacks are fully removed, like translucent armor falling off so CC-ing becomes more fluid.

Player debuff:
Boss’ one attack causes random weapon set to be unusable for some time.
Rooted until a combo field is made under his feet.

What do you think? I didn’t really like Canach’s and NULL’s mechanics so thought of these.

Yeah I think boss mechanics should reward player situational awareness, chat notifications don’t do that at all. In fact the developers implemented the chat notifications deliberately to make boss fights easier, that’s the wrong direction to take things.

I like defiant stripping animation idea.

Mechanics that rely on conditions should work with any condition, because not every class has access to every condition.

Invulnerability:
Walk in the park and mostly ignorable for some party composition.
Very hard to impossible for others.

Boss debuff:
Better version already exists, see dredge fractal and underwater fractal.

Player debuff:
Did you forget there are Elementalist and Engineer in this game? Either they get to ignore this debuff or you go and break class mechanics. Still shares the same problem as Invulnerability mentioned above.