You are right about there being a good number of elemental attacks by monsters, but I still feel that having damage mitigation based around a single damage type may not balance well against the more generalized damage mitigation of the other tanking classes. It would be cool to have abilities that play off of better damage mitigation against magical or elemental attack though.
As far as MP acting as a damage mitigation resource, you are definitely right that it would be more difficult to balance, especially with things like the bard ballads. I still like the idea of managing another resource to provide your mitigation simply for the fact that it could be made to play and feel very different from PLD and WAR. There in lies the challenge, figuring out ways to make such an idea work, but i think it would be worth it![]()