indicators at the time:
stunnable (low blow, shield bash, leg sweep) -> no indicator
knockback immunable (surecast, arm's length) -> no indicator
cast interruptable (head graze, interject) -> cast bar flashes red
debuff cleansable (esuna, the warden's paean) -> white bar below debuff

I guess new players do not even know those indicators.

I suggest for interruptable and cleansable:
if the enemy/player target could be interrupted or cleansed, the skill on the skill bar should highlight in some way to add another "push me, I am useful" use. (make it flashy? "combo-dashed-line-animation"?)

regarding stunnable and knockback immunity I have no ideas. probably not something to look in, cause in a fight either they would flash up all the time or none of the time, which is not helping the situation.
oh right there is heavy, sleep and bind too but neither ideas.