Well, the OP is making a separation between "tanking" and "keeping hate". According to him/her "tanking" is mainly about taking damage. That's why the OP wants enmity abolished. Because stacking a bunch of enmity stuff is getting in the way of stacking HP/Defense.
If you got rid of enmity and put in an ability to keep hate, all the tank would do is try to minimize damage and add loads of HP to his gear for survivalbility. It would make tanking a bit less complicated when it comes to gear and materia. It wouldn't make it easy to tank.
Of course, another way to make tanking less complicated is to improve the stats on PLD AF gear to actually INCLUDE HP and Defense and buff the class itself so that PLD has a lot more HP and Defense than other classes and then we would have to Worry about what materia we put on our belts and weapons....
But that woud be too easy.