They kinda did it that way, the Hammer taunt is a dash + small damage reduction, the Shotgun taunt is a large damage reduction, and the Chaos taunt is an impair and purge. (hammer’s 2nd taunt is in a completely different boat - it generates no aggro, only forces the target to attack during the cast)
It’s a little pointless making the button do 2 things though cause you’re not locked into your abilities between fights, and fights that require a taunt need every cast of that ability to be used for the taunt. So at that point you’re picking which one taunts most effectively, which is chaos>shotgun>hammer. If you have the target selected and you’re <15m away chaos will always taunt. If you have the target selected and you’re <5m away shotgun will sometimes taunt. If you have the target selected and you’re between 3m and 15m away hammer will sometimes taunt.
Small damage reductions (eg. most extra talis but also majority of weapon passives) don’t really have a place because healing is an all or nothing game, you either heal/mitigate fully every cast and win or don’t and die in 5 seconds. They’d have to cut back on monster damage and also healer effectiveness to change that dynamic. And that’d be much harder to balance across all the content than the current “healers/sustain tanks can take care of all damage”