Two Tanks?

[Abriael]

We already have "magical" tanks in the game thanks to the armory system. I, for one, use a lot of magic to increase my aggro generation and resistance (heals, absorbs, bio and such).

With how the class system is implemented, you can easily decide to be a more physically oriented or magically oriented tank at leisure.

In any MMORPG the number of tanks needed for each group is much more limited than that of DD/Healers. Increasing the number of tank classes risks to overpopulate the tanking role, like it actually happened in FFXI.
Considering the fact that a LOT of people play Gladiator (and many end up having to double up as gimped damagers) I would say adding more tanks is unwarranted. Especially considering that variation already exists.

This, of course, not even counting the fact that we already have multiple jobs with tanking potential in the game. On top of gladiator both Marauder and Pugilist have tanking abilities, and can tank very well with the right ability setup.