First statement: I haven't raided since coil, since alex doesnt interest me, and i'm just going off what i've tanked casually and read on the forums. I'm hoping this is a fun change to the 2 tank parties that may see some use, but maybe the skills will end up being superfluous.
So, the new tank skill sets seem to have more synergy with your co-tank, I'm wondering how much of a difference this is going to make. First off, the Blackest Knight will more often than not go on the MT, whether thats the drk himself or whatever is MT at the time. we actually get to help mitigate damage on the MT with no dps loss! (if im understanding the skill correctly.) Generally I'm assuming that a PLD won't need this so much unless the raid tiers are really beating tanks into submission, so much so that a rampart shelltron alone won't stop it. It also means it may be safer for the MT to tank in tank stance, since for some of the blows they'll be having a shield every time the drk gets there in their rotation. Even having a WAR MT and drk OT is a little better for it.
PLD, seemingly the go-to MT this expansion, is getting some extra benefit from cover, since that gives the pld themselves a 20% defensive cooldown. It will also give them some MP restore with shelltron if they're not being hit. This could go on the co-tank, but also probably a nearby melee dps. It also now has intervention, so if the co-tank is currently handling some enemies or mechanics, there's a bit extra defense there too. Oh and if things are hitting much harder in sb than they otherwise would, at least a paladin can help a drk MT swapping out of grit to hit blood weapon.
And of course, Warrior still brings the tried and true slashing debuff, though its probably more likely that this is going to come from a sam or ninja in the party, especially with the high numbers of samurai that will cme from it being one of the shiny new jobs.
Apart from that i don't see much with warrior. it being in the OT slot allows it to take less damage and get more effect from upheaval...and they don't have the 10% damage down anymore? I'm not sure if the rotation is the same but at least shirk will allow a war to hand over more aggro to their MT without losing damage themselves.
So question is: How useful do you think this synergy is going to be? WAR seems a bit more focused on its own dps, and the other 2 seem to think that adding even more damage mitigation will be handy. With the fact that currently a lot of people tank outside of tank stance, will all the added mitigation with stormblood be useful? I don't raid myself but im assuming that they are going to have to increase the outgoing damage from bosses to the point where all this is necessary. Or maybe its just to give healers less to heal and more dps to do?