Well, that's wrong.
It's easy to find recent games where emity is a thing, even in Final Fantasy. FF XIII with the Sentinel role, FF XII with the Decoy skill, FF X with the Provoke ability...
You could go even further with all the Cover like abilities. In fact, while showing the origin of Calacabrina to my girlfriend (To understand the reference of patch 3.2), I've found a low level FFIV run where the player kept every one at critical HP to ensure that any physical attack will be targeted at Cecil.
And the recent 3D version of FF III also gave Provoke to the Viking Job.
As for Mitigation, Guard is also an old skill in FF, and FF III pushed this even further by allowing a character to dual wield shields.
Yes, most mages only get robes. But switching to "Mages are not frontline characters because they only wear robes" to "RDM is the tank because he can wear light armor" is, maybe, skipping a step.
To wear a shield, all you need is a one-handed weapon. It's sad to assume that having a one-handed weapon will automatically make you a tank.
Do you realize, that, for now, FFXIV follows that idea as much as it can ?
Yes, we don't have a proper "Temper"...that's what I said when I mentionned "self targeted only". But giving the ability to increase the target's attack power would exactly be something new and valuable.
Yes, you are. And I'll also repeat myself. Tanks are already pretty hard to balance, since the game lacks complexity. Having a tank that can survive, do damage, and heal will, again, make it a direct contender with PLD.
(And if you look at FF history, those jobs are indeed pretty close to each other, by both being "Frontline jobs that can heal")
And since survival is automatically enough and healing is not meaningful (for a tank), then the tank that will be excluded will be the one that deals less damage, be it RDM or PLD.