Radical thought...remove the off-tank?

[RiyahArp]

Considering this would be for roulettes and easier content, why not? It worked for raids. Just leave it for hard content. This wouldn't solve the main problem though, leveling roulette.

[BillyKaplan]

It worked for raids.

Even after the changes, you still have 3 tanks per raid. That's more than enough redundancy and add-management while six tanks was overkill. The only place where 2 tanks per party might come in handy is on Ozma because all 3 tanks have to pull their weight and losing even the one means a potential wipe if you don't recover fast enough (all things said relating to when that raid was on-level, on-gear), but that's about the one place I can think of. Even in Void Ark, one tank grabs the tree, the other grabs the puppy, and the last one hits DPS stance unless one of the other tanks dies.
None of this applies to 8-player content where the 2 tanks is all you ever had and you don't have other parties picking up your party's slack. And that's before we get to 'easier content' like A9 where if you remove the OT mechanic you might as well scrap the fight because that's all the mechanics you have in there, the add management, and you need the OT for that.