The problem is that SE backed themselves into a corner with how they upgraded Aegis, and with how powerful Ochain is. PLD is indeed, completely superior in pretty much every way as a tank to any other job that can tank.

Short of nerfing Aegis into the ground, there isn't really a whole lot they can do with RUN, it seems, to actually make it do what it was sold to us as being capable of doing. RUN gets doubly screwed in this, as the lack of a shield severely hurts. It's kind of telling that the RUN Ergon Weapon has PDTII-25% on it. That's called a bandaid for the fact that RUN can't use a shield.

Here's one suggestion that no one would like but would probably be thematically sound, in keeping with what I see of RUN: Double the number of runes that can be active at once to six, and halve their potency (so that a RUN with six of a single type up is effectively the same). Then add a bonus for keeping a certain progression going. You put up, say, Ignis, then Unda, then Sulpor, then Tellus, then Flabra, then Gelus, and continue the rotation in that fashion, you gain increasing -MDTII and resists to all those six elements. There could be other combinations, like Ignis/Sulpor/Flabra/Ignis/Sulpor/Flabra, wherein with nothing that is weak to the others, you get enhanced bonuses to simply those elements, but higher than otherwise. Put up all six elements in rotation, get a small, but constantly increasing while you keep the chain going boost in MDTII and resists. Do the same thing with any two/three elements that are not strong/weak against one another, and get a similar, but better, bonus to those elements.

The way the elemental wheel works out, this would mean that you could, starting small and building up over time, boost everything, or boost the light- or dark-aligned halves of the elemental wheel at once, or you could do Light or Darkness.

SOMETHING that has to do with how you combine those runes, like with how PUPs have to pick and choose and cycle their maneuvers to activate attachments, would make it slightly more engaging, and would serve as a way to push the job closer to what it should be, in terms of mitigation.

Another option is to just bump RUN's MDB up higher. Or maybe just give them innate -MDTII, since that, stacking additively with MDT, and those two being multiplicative with valiance/vallation, would be a huge damage reduction to elements weak to the harbored runes.

However, that would piss off all the people who have Aegis.

Or wouldn't matter, knowing the community.