I just realized that all the heroes can be stunned. It might be debatable but I feel at least one hero should feel like an unstoppable force of menace.
Bronze, stone skin: golem becomes unstunnable and cannot be knocked back for 10 seconds, granting himself a shield
Silver, rock solid slam: gives himself immense armor and readies himself for a powerful elemental attack
Gold, unkillable: the first time he would be killed he is not, and remains invulnerable for 10 seconds, healing himself for some amount
Platinum: force of nature, whenever an opponent is stunned this hero gains bonus basic weapon damage for a limited time