Dark Knight
We all know DRK, and to be more specific Edge of Shadow is pretty problematic right now, but I feel like most suggestions to the skill tend to go a bit far. I don't agree that the skill should deal less damage, or be nerfed to 10% instead of 20%. 10% is basically irrelevant, at that number it's almost not even noticeable whether it's reducing any healing or not. If we wanted to keep it just as spammable, then maybe it'd be an alright path to go? But I think the bigger issue is how often the debuff can be applied more than anything else.
I think the MP cost of EoS should simply be increased, enough so that passive MP regen is not enough to fund it so you have to make conscious effort tom keep your MP pool at a good amount. With TBN being 0 MP, all your MP goes exclusively to EoS which makes it far too easy to manage. I do think TBN should be free, because now that every tank has a party support button it would be unfair for one to arbitrarily have a cost while the others are free and on the same cooldown.
I think increasing the cost to perhaps 3300 or higher would go a long way to putting the job back in line. At 3.3k the ability can only be used a maximum of 3 times in a row, and enough of a cost that you would struggle to keep uptime on the Darkside buff unless you made an effort in your MP management. The MP cost could even be a bit higher, to account for passive MP ticks (250 roughly every 3 secs). In two souleater combos you'd get back about 1.5k MP when taking into account passive MP regen, more if you had a bloodspiller held. But I feel 5k or more would be far too restrictive with the way the ability works now, so I'd say 3.3k~ is a good spot. If the MP cost were increased any further I'd say the duration of the debuff should be higher to compensate otherwise it would be too weak.
As for the debuff stacking being problematic...The devs would have to change how stacking currently works otherwise it wouldn't be consistent. Damage up and damage down effects stack, like caster LB + inner chaos + feint and whatnot, which are already strong effects than healing down. To have them not stack would be confusing and inconsistent. I think the bigger issue is how consistently available the debuffs are rather than the effect itself. When it comes to damage up effects they're locked behind cooldowns (as is the case with feint, phantom dart, caster LB etc), so if you hit the availability of EoS and Biolysis I think it'd be the proper route to go.
I really like the way EoS works and I feel like it'd be a shame to gut it or change the way it functions. More diversity is good, and if anything I feel like the other tanks should be brought up to DRK's level rather than the job be brought down.
Warrior
Honestly WAR is in a pretty good spot, it's just in comparison to DRK it's not as good. If any changes were to be made to the job, they should be small ones rather than anything major.
I like the idea of Onslaught being a stun again, that on its own would be really good (working under the assumption that all tank stuns get changed too, with low blow being removed from WAR).
I also think IC should generate some gauge again, WAR's personal damage feels lacking and I understand that it's balanced out by the fact that IC increases your party's damage, but the damage still feels lower than it should be.
Also i think most of the WAR playerbase can agree that NF should be self castable. Using it on yourself without a party member is a buff for rare situations where you're by yourself, but is actually detrimental in party play. You'd be losing out on half of the skill's effect if you use it just on yourself, so I feel like it's not really a buff but more of a tradeoff. (It should also be self castable in pve, reeee)