I disagree with almost everything you said. For example: PLD and WAR have the same exact rotation. you do your main rotation, you hit your buffs, you do your special ability spam, in WAR's case it's fell cleave, in PLDs case it's holy spirit. Even with Royal Authority/Sword oath, it still feels the same. There are some minor use case differences between the two, but more or less they are mechanically the same. So similar that my PLD and WAR chotbars are setup identically. DRKs and GNBs are sort of similar, in that you have build-up and the you spam ogcds between your main rotation. Mitigation ways, DRK also has another difference from the other tanks, in that they have magic mitigation. (as in, does not affect physical damage) which makes DRK superior defensive-ways, but only sometimes. War also has a differing mechanic as it relies heavily on HP recovery and HP manipulation as mitigation tools. While the other classes have similar mechanics in them, none of them replicate what war can do with it's tools.
In heavensward Each tank had 3 variants to their combo. One mitigated damage, PLD physical, DRK magical, and WAR all damage. The fundamental problem of this was that *some* players would forgo that particular rotation in order to maximize damage, and then pin it on the healer for not mitigating properly. On DRK SQEX has mitigation tools side by side with DPS tools, making the player decide which is best at a given moment. (TBF a lot of ppl actually use DRKs stackable mitigation. that one worked real well) But if you keep down that path the tanks different enough from each other. If there was a tank dedicated to just that active mitigation idea. A basic rotation, and your ogcds were mitigation. It'd be different enough, and maybe just interesting enough, to be a competitively fun tank, as well as different.
With your active mitigation idea, we actually have an example of this outside of DRK, though it is based on MP instead of stacks. It's PLD's Clemency. By timing your clemency so that it goes off immediately after big damage does, you can save yourself from a lot of pain. (Is it optimal? no. But it's worthwhile to at least know how to use it.) It's what separates Big Deeps PLD from Raid Saver PLD. The flaw in SQEXs design philosophy. By making the content easy enough everybody could do it with enough dedication, the community tends to lean more towards DPS than not DPS. But every time they implement mechanics like these, it's at the cost of DPS.