I'd say give WAR get more self-healing again. Or make Defiance's healing bonus work with healing abilities while nerfing Equilibrium's base potency to factor in the bonus. Hell, make Equilibrium restore TP and heal in both stances but give more TP in DPS stance.
In general I would like to see more reasons for tanks to be in tank stance. One thought I had to 'force' it without feeling bad necessarily is to make the DPS stances give aggro reduction, and to reduce the damage penalties of tank stances in general (PLD and DRK in particular could use it more than WAR). Raids would obviously need to change to support this with higher auto attack and buster damage but I agree that if this happens then we need to see all tanks get oGCD stance swaps. It shouldn't cost any resources to do either. Sword Oath may need to get looked at because I can hear people getting mad about dropping free auto damage from swapping in/out already.
Giving tanks more ways to mitigate damage via short cooldowns would also facilitate this but I feel that stance swaps are a better way to go about it. PLD has a lot of ways to do this already so they just need more oath gauge generation and a DPS way to spend it while in sword oath. Perhaps as a way to spend Shield Swipe? Or a new skill that replaces it in sword oath? DRK has TBN but realistically they need more. Probably as debuffs as many people have mentioned.
I kinda feel Shake It Off doesn't make sense in WAR's kit despite being so good to have. I'd consider reworking it as part of a utility trim to make WAR less desirable.
I dislike the knockback immunity skills in general given how they trivialize mechanics and the different cooldowns they all have. Arm's Length is probably the most balanced in that regard. I'd say every class should have access to that and all of them should be 60s cooldowns. Yes, even BRD and MCH should get it. Tempered Will could be the tank Role Action equivalent for sure.
Finally, make Requiescat a ranged attack. PLD doesn't need a gap closer if they can get their Holy Spirit spam window in without needing to be in melee range.