Clemency doesn't really count as active mitigation, though, because (1) it isn't really necessary to survive, and (2) it isn't exclusive to the PLD using it. Don't get me wrong; saving a group that's going south with some well-placed Clemency casts is one of the most satisfying things in the game, but it doesn't count as active mitigation. Now, could SE retool PLD such that self-casting Clemency becomes one of the most central components of its gameplay? Certainly, but I don't think that would really be that engaging.
Defensive cooldowns != active mitigation. The FFXIV model of mitigation centers entirely around ordering and managing your cooldowns, and in high-end content around stacking them appropriately, but that method of managing incoming damage is neither engaging nor fun. Most tanks do have some form of active mitigation in the form of self-healing based on certain attacks in their rotation, but it could be removed from the game and no one would notice.
To give an example of what I'm talking about, I'll describe how I remember Blood DK working the last time I played it (roughly - and my memory is shoddy af right now due to the state of the world at large). Over time, you slowly generated runes. Spending those runes gave you rune power, and you could then spend that rune power to create your Blood Shield, which was a massive shield that sat overtop your normal HP pool. A well-geared Blood DK could tank entire dungeons without needing healing once, and this was a big deal because healers didn't really have any oGCD heals (outside of an "ohshit" button or two). But the entire blood DK gameplay was built around the idea that, in order to survive the dungeon and do a good job at your role, you needed to pour everything into your Blood Shield and keep up as best as you could.
As it stands, there are a few clear obstacles that would need to be overcome in order for FFXIV to be able to entertain the notion of active mitigation as a design paradigm. Tanks would need far less passive mitigation and far fewer defensive cooldowns, healers would need to be nerfed, oGCD heals would basically need to be scrapped, and instances would need to be designed in such a way that tanks actually needed to care about incoming damage from mobs and bosses. Unfortunately, with the 2.5s GCD in this game, I'm not sure that such a design could actually work (WoW has the benefit of having a generally ~1s GCD, which makes the gameplay much more fast-paced).