Why does MT/OT have to be about damage output?

How about this:
Main-tanks: Get a trait at 30 that decreases damage and increases defence, and a cooldown/ability that disables this.
Off-tanks: Get a cooldown/ability at 30 (or starts with it for Gunbreaker) that increases their defence and decreases their damage.
(nuance can differentiate between main tanks or off tanks further, e.g. perhaps MT1 has a 60s cooldown that boosts DPS, while MT2 has a toggle-able ability that merely nullifies the dps penalty of the trait)

This would result in two different ways of tanking, one that is basically tank by default, but requires action to leave tank stance, and one that’s more dps/mitigation-support by default and requires action to become a tank.

For example, If Paladin is a Main tank, then Shield Oath would be a trait at level 30, and Sword Oath would be an ability.
Darkside being traited and Grit as an ability would make DRK an off tank.
Then I think WAR would be better suited as a main tank, with Defiance as a trait and Deliverance as an ability. (otherwise, unlimited Fell Cleave)
Which would leave GUN as an off tank.

This wouldn’t necessarily mean that main tanks need to main tank, meta could still evolve towards optimal dps, (unless the default aggro for a main tank makes it difficult to off tank, like a PLD before getting sword oath) but it would add some variety to the way tanks play.
For example, DRK/PLD could result in DRK bursting with Grit for aggro, and PLD using Sword Oath on cooldown and shirking every now and then. So MT and OT would be swapped.
Or, maybe WAR/DRK would have WAR main tanking, because its burst with Berserk has the greatest aggro gen, and then it can still burst with Deliverance for its Fell Cleaves, and DRK simply DPSes without touching Grit, using TBN on the WAR. This would be MT and OT in their proper places.
So MT and OT would be more about playstyle, than actual role in a fight, which would still be flexible.