This is a suggestion based around maybe finding a way to make a real tank actually have a place in the game again. So, what this suggestion is NOT, is "make everyone need more fortification". Adding one more thing to the list of junk you need to slot isn't the point, so the IMPLEMENTATION of this idea matters VERY MUCH. Also, it's a holistic suggestion, in that any part of it on its own is going to seem stupid or obnoxious because it depends on the other parts to make it work. Anyway, here it is:
Step 1: Reduce basic mob damage across the board in ALL difficulties. This is going to sound nuts. Cue the "what, you want to make the game trivial?!?!" rants. No, that's not what I'm getting at here, because of . .
Step 2: Give mobs--ALL mobs--more and bigger critical/sneak attack damage. And GIVE THEM SPELL CRITS if they don't already have them. Ideally, you want to hit a number where their overall "dps" is not really decreased much if at all (and may even be higher on someone with zero fortification) but the vast majority of their damage, as it is with players, comes from CRITICALS and ADD-ON damage like sneak attack etc. You may even want to classify the DOT effect some champions get as "critical" damage for this purpose. Also, you could get creative and hand out more stuff like Bleeding dots on rogue mobs, that sort of thing.
Step 3: make Fortification also protect against SPELL CRITICALS.
Step 4: change Fortification to not be an "all or nothing" system but instead reduce how MUCH extra damage you take from crit damage sources (sneak attack, crit multipliers, maybe DOTS).
Step 5: give tank-focused trees/builds access to MASSIVE amounts of fortification in their defensive benefits, but it requires investment or trade-offs, like you get 3x as much in heavier armor, most of the fort is bound to using a shield, etc. You *can* get to a goodish position on a non-heavy armor toon, but it's going to require some compromises. For Reaper, add some fort to the Guardian tree. Instead of increasing difficulty by massively scaling up raw damage, scale up fort BYPASS. Don't increase difficulty by massively scaling down player raw damage, scale UP mob fortification/prr. (And put some fort/prr bypass in the Reaper DPS tree, because that tree is a joke.) And don't give casters ANY crit bypass on spells. You know what caster spell crit bypass is? FINGER OF DEATH, that's what it is.
So, why do all this? Because mob damage in higher difficulties (reaper/legendary raids, etc) is, frankly, stupid. If you build a "tank", the only difference is--maybe--"the mob two-shot me before anyone could get a heal off instead of one-shotting me before anyone could get a heal off". It doesn't actually keep you alive any better, and you're just as dead if you get threat as anyone else. This won't really change anything for non-tanks . . . they'll still be getting hit hard and squished on a regular basis. But tanks will have multiple layers of defenses. The basic damage will be mostly absorbed by their PRR, and the bonus damage will be, again, mostly absorbed by their fortification, so the resulting damage that gets through will be in a reasonable range. There will be trade-offs again . . . you need Fort AND PRR, but there's a range between "good enough" and "ultimate" . . . even the "ultimate" won't ever reach an "I ignore all damagez!" stage, but at least they'll be out of the "fastest finger heals" competition (or, more like what it is now, the "fastest finger resurrection"). Everyone can get SOME fortification that helps them stay alive much better, but if you're aiming for Big Damage and don't want to be able to tank large numbers of mobs or bosses, you aim for "good enough" instead of "ultimate".
Isn't this kind of like PRR all over again? Well, yes, pretty much. AC was too much all or nothing, so it was put on a diminishing returns system, dodge was introduced, and PRR was introduced. And it was actually working well for a while, until ya'll went and MADE DAMAGE STUPID AGAIN. I don't mind having a frank "trololol" difficulty, but let's even out the trolling a bit, here. By all means, leave it stupid for everyone who isn't invested in tanking, in fact, I encourage it. But cut the tanks a break already, sheesh.