The problem with the OP is they see a system in another game, think, I like that, put in FFXIV, but never think about how that affects FFXIV. Just slap a random system in there and hope it fits.
But it didn't really work in the past. If we go back to HW, DRK was the magic tank and PLD was the physical tank. With the majority of damage, especially raidwides and tank busters, being magical, it skewed the preferred tank to DRK (even before you take into account DRK's higher DPS).
Now, bringing things back to more recent times, if we were to adopt this idea, there are 2 ways it could be done.
1. Split the tanks and have 2 physical tanks and 2 magic tanks. This would, in theory, mean one set of tanks is better than another in a given fight, however, we also know that fights are designed so that all tanks can clear all content. This by necessity means the physical tanks can mitigate damage in the magic heavy fights and vice versa. This creates a potential...problem. If the extra mitigation power doesn't prevent someone from using a resource, then that extra mitigation is functionally useless, if it does, that is when it will tip the scales in favour of the 'right tank pair'. Then it is a case of how much it benefits the party.
This also creates a problem in the fact you have Split the tanks into 2 preferred parties, one physical based, one magic based. of course, there might be fights where they mix up the damage more, so you are better off having one magic and one physical.
2. The other way is giving all tanks a mixed suite of physical and magical mitigation. Without then massively bloating the mitigation kit, you would necessarily have to split it and have a physical and magical variant for each. Take PLD for example, rather than the 5 personal mitigations they currently have (Rampart, Guardian, Bulwark, Holy Sheltron and Hallowed Ground), you would half it and make some magic and some physical. Here is Physical Guardian and here is Magical Guardian.
Going onto raidwides, you would need 2 for every job, one for physical one for magical.
Then we have the problem of shields. Using Guardian again, it gives PLD a 1000 potency shield. Shields are damage independent, so they absorb both types. This would mean tanks that had shields would get more mitigation power over all damage types.
As you can tell, more and more balancing issues come into play when you start to dig deeper. This is especially true when people want DRK and GNB raidwide mitigations to have a physical damage mitigation component (or just reduce all damage) and Dark Mind to also have something for Physical damage (just as Camouflage has something small for magical attacks). Also, bear in mind, this is for 2 damage types, I don't even want to think about the mess that would be OPs suggestion.
People might disagree with things I have said, or they don't care about any differences, which is fine. I am just illustrating things that need to be thought about when you want to design a mitigation kit for the tanks.