While I agree they could do a lot more to make the tanks more interesting and interactive, I will disagree with the idea that the tanks of final fantasy are usually the protagonist.
This is true for most(if not all) of the 2D games, so I won't disagree that the precedent is there.
However, none of the FF7+ games have had a tank as a protagonist. I didn't play 12 though, so there might be that.
FF7 - Cloud is a balanced stat character, Barret is clearly the most defensive. If there's a tank, it's Barret.
FF8 - Well, there's really no tank in this game. Squall is not any more defensive than the others. Zell probably has a higher defensive growth, but I haven't looked into it.
FF9 - Adelbert Steiner is the Knight. Zidane is a Thief. No question here.
FF10 - I'm really surprised you could say Tidus was the Tank. Auron's HP growth was insane. He would live through anything that would kill the rest of the party.
FF12 - Didn't play.
FF13 - Lightning is not a tank. Her growth capped as a ravager, she made a decent commando and medic as well. Overall follows the "balanced stat hero" that the 3D games follow. Snow was easily the best Sentinel and a clear tank.
This may seem like nitpicking but it does argue against the dps increase being an iconic tank FF tank trait. Being at the front of the party in cutscenes doesn't make that person the tank since the actual battle didn't push defense on those protagonists while it did on a character next to them.
That said, I do think you can make the tank feel like they're leading the charge in a different way.
I think active mitigation is really a better solution than adding DPS. Tanks should have a more fluid system of defense that they have raise or lower in preparation for bigger attacks.
To make it more final fantasy-esque, I believe defensive abilities (some) should affect the whole party. The only way they can do this is through the LB, but because it is shared few people ever use the tank's LB. Tanks in final fantasy usually have more ways to reduce damage to the party.
Sure the boss is attacking you, but nothing really says you're a guardian like raising everyone's defense and telling the boss that this party is under your protection.
Of course damage phases would have to be designed around that and healers will still be needed. In fact the only difference for damage taken from how it is now would be that if the tank fails at timing, it will be harder on the healers and it may lead to a wipe. But that's part of party synergy and putting the defense of yourself and the party in the tanks hands would attract a lot more people to it imo.
Threat and damage are really not what tanking is about. Even (or especially so) in Final Fantasy.
I do encourage damage as a supplementary part of the role.
I don't think threat should be hard for tanks either. Some people may think that's an important part of tanking, but I think a lot more think it's just annoying to have to chase after a mob.
I think personal and party defense should be the bread and butter of tanking. It should be active and it should be what you're manage throughout the fight.
Warriors are something like this, but it's only on a personal level. As a tank, you've inherently got a spotlight on you. Not everyone likes the spotlight.
Add to that, people don't see your success much but they will definitely see your failure. I think it's obvious why people don't play tanks much.
(So resisting making a joke about it being a "tank-less job"....oh, I guess I did anyway)
If you're going put the weight of the party safety in the tanks hands, I really think they should be able to show off a little more how they're helping everyone.
Tanks should be the sentinels and FF is known being able to show such things with style. However the most notable defensive move is the limit break while few others show anything at all.
As a dps, your abilities still give you the feel that you're taking down the enemy. As a healer, you get spouting lights as a whm or creating defenses as a sch.
The tank is usually physically away from the rest of the party, because the boss is between you and them. Little things like that really affect people's perception.
It would go a long way for you have abilities that link you to the party, so tanks don't feel as much like they're having an argument with the boss in a corner while the rest of the party is well....partying.
I.e. raid cooldowns.
Active mitigation with personal and raid defenses would make tanking much more appealing.