Time BattleMage (job suggestion)

[Tzain]

I see your point, maybe making it the least bulky and average damage with the bulk of the tanking coming in the form of time magic with making the enemy attack slower and you faster(increased party rate)

Ever play WoW Monk tank? They have an ability where when they get hit the damage is applied to a debuff that does damage over time. So they spread out their damage over time to be more manageable.
That *might* be workable for a time mage to manage their incoming damage.

I'd be hesitant about tanking through Slow and Haste though, or really any debuff. Because either the ability is mostly useless or it immediately becomes mandatory since the Slow could be applied while the Time Mage is the off tank, thus making the main tank even more tanky.
The meta would have a freaking field day.

I think when you consider the "mitigation style" of the tank, you have to consider it to be an internal thing, not an external thing for that reason. Utility would be the external consideration, so while a Slow Debuff would be possible, it would have to be the "Utility" of the tank and not it's primary Mitigation.