I'll edit in my post then. Thankfully, I wrote it up ahead of time anyway so already had it. Using spoiler tags to separate the fluff from the changes.
Let's start this off with 1 thing. I am not a game designer. This is 100% player perspective and as such there will probably be issues I am not aware of. I will also for this reason avoid calling out direct potencies since I also do not have the overall knowledge of the delicate system that is game balance. I will also not be naming skills and statuses because I simply lack that kind of creativity.
Why I think DRK doesn't feel as good as the other tanks:
That being said, let's first talk about why DRK is so widely disliked. Living Dead is an obvious one, and while I feel this should be reworked, I generally consider it a smaller factor in why DRK is so unpopular. More importantly, the Blood Gauge is literally just the Beast Gauge and Delirium is basically just Inner Release with MP instead of Direct Crits. The Job doesn't have any real situational uses for any off globals which makes it entirely "use x when available" while only have a 1-2-3 combo. The lack in a self heal in any form aside from the main damage combo makes it feel very reliant on outside sources with little to no actual raid utility outside of TBN. Next to the other tanks, it just feels like it has no identity aside from being Warrior, but noisier and worse. So before I fix it let's compare its identity to the other tanks and why they feel better to play. When I compare tanks I will be including all Endwalker skill changes since that is right around the corner.
Paladin is a good place to start. PLD also gets MP regeneration on part 2 of their 3 part combos, but they have the option between a hard hitting skill and a DoT and ideally refreshing the DoT as it expires every time. This makes the classes basic rotation feel interactive as you want to watch your combo timings to maximize the DPS from the DoT and Goring Blade. The DoT is then best applied just before Requiescat which will be a main source of damage. PLD feels good because it's basic combo is divided into 2 combos and its burst phase is based on an off global that will soon be based on charges. PLD has many ways to reduce party damage as well as Clemency being an incredible heal, meaning it also feels good to be a Paladin when things in the content aren't going too well. Everything you do is independent, useful, and consistent. DRK has similar if not better MP regeneration regularly, but unlike Paladin, it only has one thing it can do with its MP and its DPS output is heavily reliant on how frequently it uses mana.
Warrior is the one I admittedly know the least about. Its different materia melds have left me letting it fall behind my other tanks for usage, but I think I can still make a fair comparison. Equilibrium is a great self heal, and soon to be AoE heal over time, which is always beneficial to have since it presumably will stack with other overtime heal effects. The burst window for Warrior when perfectly executed is a lot of damage, and having a combo that keeps up damage and a combo that builds more beast gauge makes it feel interactive in the way of keeping your damage up. The class feels good to execute without being too complicated. The raid utility focus a lot on healing rather than damage reduction. DRK plays very similarly to the warrior, but also has less on global cooldowns to use with more off globals. Warrior feels clean and forward, but DRK seems noisy and hectic by comparison.
Gunbreaker is next, and this is one of my favorites. Gunbreaker is the only other tank that only gets a 1-2-3 combo with no alternative. The second part of their combo heals and generates a shield while the third part generates a catridge which is used to pull off several actions. Every GNB off global is essentially used as soon as it's available, just like DRK. What separates GNB from DRK in this regard is that no matter what happens, after you execute your basic combo, you have another option available to do damage that is on your global cooldown. The 1-2-3 combo is redundant and repeated, but it feels different, it feels like a setup to something larger. It's also the only tank with a DoT that cannot be refreshed upon its expiration. GNB is our proof that the problem with DRK is not the basic 1-2-3 playstyle, but rather the feeling of what you are progressing towards. GNB makes the single 1-2-3 combo work because every single time you execute it you build up to something better on your GCD to break up the monotony.
And then there's DRK. You spend the entire encounter pressing 1-2-3 until Delirium is ready and then using Bloodspiller over and over until you are out of free uses and blood gauge. All the while you keep using Edge of Darkness to keep up your darkside and you have The Blackest Night, which is probably the best tank mitigation skill that can target someone other than the actual tank. Breaking TBN gives a free cast of Edge of Darkness. But aside from that, it has no real identity. If PLD is the protector tank, War is the heal tank, and GNB is the DPS tank, DRK is the middle child who wants to do it all but ends up losing sight of what it actually should do. As it stands, DRK actually isn't that bad. It's a class that functionally works and for this reason I personally don't see the need for an entire rework to make the class feel good to play again, I just think it needs to take a lesson from what makes the other tanks work and become something more interactive and less tedious.
The changes I'd propose:
This is where I finally start to propose some changes. Let's start with the obvious glaring problem of DRK that is Living Dead. I fully believe the idea behind living dead, a tank that cannot die until it is healed, is a wonderful idea for DRK. It fits thematically. the problem comes from self sufficiency. While PLD is the only truly self sufficient immunity, War and GNB still can hold their own and help the healers get them healthy again. DRK is the only one that looks at the healers and says "This is your problem now" and then if they succeed your immunity goes away early. It's the only immunity that is completely reliant on the healers alone and punishes you for the healers success. My proposed fix is as follows.
Living Dead- Upon activation gain the living dead status. When you fall to 0 hp you instead fall to 1 hp enter a state of Walking Dead for 9 seconds. While under Walking Dead you cannot take damage or recover hp. For every 3 seconds and every weapon skill used you gain 1 [stack]. When Walking Dead expires, heal for [potency] per [stack].
This takes the reliance off the healers and allows them to worry about the raid as a whole as opposed to the tank who killed himself, and would probably still require some after-the-fact healing while giving the DRK enough hp to take a hit or two after it expires. Not being able to be healed sucks, but so does Superbolide dropping you to 1 hp just to skip a mechanic, so I think it's fair. Furthermore, if you have to use invulnerability in a situation where you cannot hit the boss for any reason, you still gain at least 3 stacks to get enough of a heal that you aren't doomed. Ideally, no amount of achievable stacks should fully heal you. Regen buffs can still be applied, just with no healing starting until the effect falls off. It's then the DRK's responsibility to decide if they need to use TBN, since shields still take effect. If possible, it would benefit from being coded that Scholar shields still apply as well.
But let's get into the real meat of it. Let's really fix what it feels like to play DRK.
Blood Gauge and Delirium I think can actually stay as they are. The real problem DRK has isn't being a copy-paste Warrior, each tank does take something from the other, and I think having 2 tanks that have a "big hit many times" button is actually pretty cool. Instead, I propose we bring back Dark Arts, but actually bring it back as opposed to being a free cast of Edge of Darkness. To be honest, I think we can fix Dark Knight by adding just 3 skills which would entirely change the way DRK is played and the way it feels. If Dark Knight is the class that wants to take a page from every tanks book, I say we let it, but really lean into that identity.
[On Global Spell] - Only available while under the effects of Darkside. Channel your inner darkness to deal [potency] unaspected damage. Consumes 15 seconds of Darkside time. Does not interrupt combo actions. (Maybe name this Dark Passenger after the 3.0 skill? Could even recycle the animation.)
Dark Arts- 120s cooldown off global. Requires 50 Blood Gauge. [New Spell] no longer consumes Darkside time for 3 casts.
Sourge (3.0 Dark Players remember this) [on global cooldown] Deals [potency] and applies DoT for [potency] damage for 15 seconds. 30 second recast time.
Why I think these changes would work and final thoughts:
This takes a page from PLD that allows DRK to cast a spell many times over to deal damage, and indirectly costs MP. Someone smarter than me did the math and in an ideal fight for 10 minutes, you have 12 minutes of Darkside that is completely wasted. It's basically a free resource. This causes more thinking about when to increase your Darkside time. You want your MP up to refill it when it drops below 30s, but you want to burn it below 30s to use your new spell and prevent overcapping. Then just like Requiescat makes Holy Spirit better, you get free casts of your new spell periodically. I feel Dark Arts should not increase damage, just availability.
This also keeps its identity from Warrior in the Blood Gauge and Delirium. Warriors Inner Release applies more damage, but since Delirium is granting MP this means more Darkside uptime which also means more casts of the new spell. Keeping Delirium as it is is also why Dark Arts shouldn't let the new spell do more damage, 2 increased damage rotations would be difficult to manage and balance in many situations.
A DoT that works much like the GNB DoT in the recasting of it being different from the duration means you have another action to break up your 3 part combo. But importantly, since your combo has MP drain that means that after every 1-2-3 combo done, you can potentially gain enough MP to keep Darkside timer up, which again, may be consumed for the new spell. Each 1-2-3 combo feels like a build up, just like GNB.
The playstyle provided would involve emptying your MP into your Darkside, consuming excessive Darkside to do more damage, lead into Delirium to regain MP, then turn the MP into more Darkside that is consumed into more damage, brought back into a Blood Weapon MP regen rotation, and again consuming your MP for more Darkside which is converted into a damaging spell and charging your Blood Gauge for Dark Arts. This playstyle makes you want to keep Darkside around 30 seconds as opposed to overcapping simply because it's a DPS loss to not overcap. You would now be losing DPS by overcapping and makes your MP more important. Of course, adding this consistent DPS means something somewhere wold have to give and it might result in lower potencies in other places. My guess is Edge of Darkness would suffer, since it would lead directly into another DPS GCD.
There's nothing wrong with DRK copying things from the other tanks. This is a "holy trinity" game, so we have the trinity covered in the "tanky tank" the "healing tank" and the "damage tank" so one tank will inevitably need to be able to do a little bit of all of it. No, it still doesn't get a good heal, but these 3 new skills don't change that DRK can sometimes feel "noisey" especially in the opener. Adding a heal, on or off global, would add to the noise. The Blackest Night is just too good anyway, and the free cast of your Darkside skills would stay as well, so a heal for raid utility probably wouldn't be needed. Alternatively, if a self heal would be necessary to help the class, tying it to Dark Arts seems thematically appropriate.
Something that adds to its raid utility that people don't seem to be talking about is the fact that the new mitigation skill that reduces damage by 10% actually has 2 charges, meaning it's significantly more accessible than the other tanks similar skills, so Endwalker remains to see how good having 2 charges will actually be, though it looks bleak at first glance due to being a clearly inferior version. All charges mean is you aren't necessarily losing out when not using it off cooldown. This is certainly a good thing, so it will take experience to find out if this charged mitigation tool can keep up.
3 skills and no traits is all it would take to make this class feel good again. Instead, we got "enhanced" unmend and a damage reduction tool that makes it harder to break TBN that is also worse than the other tanks damage reduction tool. If anything, I'd like to see a change to Dark Passenger. It's the same as the GNB skill and now RDM also gets a magic defense buff as well. I would have loved to see DRK get something a little more unique. There's a big problem with giving multiple classes the same features and that is that if more than 1 can do it, one of them will inevitably do it better. That's why my proposed changes don't make delirium better than Inner Release, but rather add more depth to the class itself so that it doesn't need to be better.
Sorry if this was too long winded, but I didn't want to just throw out a few new skills, I wanted to actually explain why I think they would fix the class and why I came up with what I did as opposed to leaving people wondering what made me think it was a good idea. Let me know what you think! I know nothing like this will come true, but I can dream...