A lot of RPGs have some kind of heavy physical character whose main benefit is their high defense. Due to the fact that enemy targeting is normally random, many of these systems create some sort of "Provoke" skill, to manage threat, and keep enemies targeting the aforementioned tank. How exactly would you balance that ability?
If the skill makes enemies target only the tank, he/she would either die very quickly, or the game would be significantly easier since the tank is holding up, preventing your other characters from ever taking a hit.
If the skill just increases the chance that the tank would be targeted, by what percentage should threat increase? If it is too low, players will feel like they just wasted a turn. If it is too high, it may as well be the previous entry on the list.
How would you prefer that a tank character does their tanking?