DRK changes that would make it really fun to play

[Archwizard]

TBN is damage neutral...?

Yes. And what I suggested wouldn't affect damage at all -- the only reward I'm suggesting is a cost reduction that effectively increases survivability, which is no different from people arguing for it to HoT when the barrier breaks.
You're not locked out of using the DA spender if the barrier isn't broken in time, because you can still use it without DA and TBN doesn't eat the resources needed to cast it in the first place.

If you want me to get more specific with what I mean, here's an example to clarify:

* TBN no longer costs MP to activate, but it's on a 25 sec CD like other on-demands. A weaker version of TBN (Shadowskin?) is available at lower levels that has a lower potency barrier and doesn't give DA.
* Shadowbringer has a 30 sec CD and 2 charges, no MP cost but consumes 20% of HP when activated. A weaker version is available at the level TBN is introduced (Flood of Shadow?).
* DA reworked, now causes Shadowbringer/Flood to be cast without an HP cost, and up to two DA charges can be stored at once.
* Abyssal Drain replaces Flood of Darkness/Shadow as the AoE MP spender. Carve & Spit is upgraded to a cone attack with damage falloff on additional targets, and no longer shares a CD with Abyssal Drain.
* Edge of Darkness/Shadow now restores HP whenever it deals damage.

MP is used for draining attacks to heal you just like PLD's spells (which also helps recover from any Shadowbringer overspending), DA remains damage neutral because it's not on an MP cost or refund. You can choose to play conservatively, only using Shadowbringer as often as you have DA, or take calculated risks using the extra charge during burst (which you can quickly recover from with your MP spenders), and how well you thread that needle could become a point of skill for the job at high-end play.

And disclaimer, I want to make explicit here that I'm only defining this example to clarify what I mean, not as advocacy for a specific path the devs could or should take. I'm illustrating just one of many possibilities.

[Mikey_R]

* TBN no longer costs MP to activate, but it's on a 25 sec CD like other on-demands. A weaker version of TBN (Shadowskin?) is available at lower levels that has a lower potency barrier and doesn't give DA.
* Shadowbringer has a 30 sec CD and 2 charges, no MP cost but consumes 20% of HP when activated. A weaker version is available at the level TBN is introduced (Flood of Shadow?).
* DA reworked, now causes Shadowbringer/Flood to be cast without an HP cost, and up to two DA charges can be stored at once.

Let's just cut to the relevant bits.

You have made TBN an offensive action now. By giving you a free Shadowbringer you save your own HP but also the healing from healers and all you have to do is make sure you TBN every 30 seconds. What, you want to save it for the tank buster? But, if I delay it, I lose out on a free Shadowbringer. I want to pump out as much DPS as I can, without hindering everyone else. I don't want the healer to stop DPSing just so that they can help to heal that 20% HP I lost when I can get it for free. Even if I use Edge to mitigate the healing (depending on how many you need to heal yourself by 20%, let's assume 1), then technically, the Shadowbringer is not free, it has cost me 3000MP, maybe more if it requires more casts, and if I cannot supplement that healing requirement, it then falls onto the healer, so now you have punished them instead.

Whilst this might work thematically, please please think about how it would work in a fight. If I were to do this, what are the consequences of these actions. Is this choice going to negatively affect someone else.

Also, point to a healer that would like to see a tank's HP yoyo around as they use their kit.

[Archwizard]

You have made TBN an offensive action now. By giving you a free Shadowbringer you save your own HP but also the healing from healers and all you have to do is make sure you TBN every 30 seconds.

So what makes TBN an offensive action here is that you're using it... to save your HP. Seems legit.

Even if I use Edge to mitigate the healing (depending on how many you need to heal yourself by 20%, let's assume 1), then technically, the Shadowbringer is not free, it has cost me 3000MP

And TBN already does every time you use it.
You literally are trying to have it both ways, each time you've spoken on this you've taken shots so broad they hit the existing TBN in the crossfire.

And, I cannot stress enough, I'm not saying "It'll be fine if they do exactly this." It's just a demonstration. I'm not gonna argue the specifics on a made-up example.

Also, point to a healer that would like to see a tank's HP yoyo around as they use their kit.

That's part and parcel with playing alongside any tank besides Paladin, at least since 6.1.
Warriors have lighter mitigation than other tanks but make up for it with self-healing. When a WAR takes big hits, we just shrug and say "Oh, he'll be fine." They're also... kind of the favorite tank right now.
When Gunbreaker uses their invuln, after the momentary heart attack, we shrug and throw a lazy HoT on them.
When Dark Knights want to use their invuln now, they pretty much stop using any mitigations and sink like a stone anyway. Sucks, but we're getting used to that.

So if you told me that DRK was able to self-heal from that occasional 20% hit due to supplemental changes to its kit, on top of it having above-average mitigation, and that I'd only have to panic if they did something incredibly dumb, I'd shrug like I would with WAR.
Is it a bit of a pitfall set for inexperienced players? Maybe, but only as much as being able to hardcast 5 second-long spells on RDM, or not being locked out of opposite element spells on BLM, or... half of the other jobs available. Knowing how to manage your own resources is part of mastering each job.

[Mikey_R]

So what makes TBN an offensive action here is that you're using it... to save your HP. Seems legit.

Technically, TBN is used offensively now, as long as you can get the shield to break just before the raid buff window. The same would apply here, however, you have the extra step of messing with your HP...for the same result, or even worse if you have to get a healer involved. Plus, it is more damage because 2 charges instead of just 1. Why have this weird HP thing if, at the end of the day, it doesn't matter.

And TBN already does every time you use it.
You literally are trying to have it both ways, each time you've spoken on this you've taken shots so broad they hit the existing TBN in the crossfire.

Yes, and it gets refunded when used well. In your case, if it is used well, you get a free Shadowbringer, you will use Edge anyway so that you don't overcap your MP and you are left in the same spot. It is if it isn't used well, currently, you are down 3000 MP, in your case, you have the weird middle ground with HP and still end up with 3000 MP loss at the end. Why is there this..weird thing with HP in the middle. What really is the point except for 'thematics'. It has already been pointed out you can play as much as you want with MP and noone cares, but there really is no point in having an arbitrary middle step.

And, I cannot stress enough, I'm not saying "It'll be fine if they do exactly this." It's just a demonstration. I'm not gonna argue the specifics on a made-up example.

However, these things DO need to be discussed if the potential for a HP drain tank to even have a chance of being implemented. You need to look at the pros and cons, you need to see what works, you need to think, is this maybe too complicated or are there too many steps? This thread alone has already demonstrated potential issues and unless an idea can take these issues and present an idea that addresses these issues and can make for a balanced job, then I'm sure many would love to hear it. However, tacking HP drain to a job just for S&Gs is not a valid reason for it to be in place.

That's part and parcel with playing alongside any tank besides Paladin, at least since 6.1.
Warriors have lighter mitigation than other tanks but make up for it with self-healing. When a WAR takes big hits, we just shrug and say "Oh, he'll be fine." They're also... kind of the favorite tank right now.
When Gunbreaker uses their invuln, after the momentary heart attack, we shrug and throw a lazy HoT on them.
When Dark Knights want to use their invuln now, they pretty much stop using any mitigations and sink like a stone anyway. Sucks, but we're getting used to that.

In those cases, they are planned. The healer knows what is about to happen so they expect and can plan. But if a DRK's HP is yo-yoing throughout the whole fight, that is when the issues come in. Are they going to survive this raidwide, is that barrier strong enough, or do they need some extra HP.

So if you told me that DRK was able to self-heal from that occasional 20% hit due to supplemental changes to its kit, on top of it having above-average mitigation, and that I'd only have to panic if they did something incredibly dumb, I'd shrug like I would with WAR.

The thing is, in a raid setting, WAR's HP is helpful, but it won't sustain him. There are still so many problems inherent with HP drain that it just is not going to come about. Even if you set a threshold that you cannot use HP spenders below this amount of HP, what if you happen to drop below it just before you were about to use it. Well, you can heal yourself up and then use it, however, you are now down HP compared to if you had the HP, and whose fault would that be? Of course, it is the healer for not keeping you above this threshold. Unless you want to make DRK HP recovery so high that it shouldn't be an issue, at which point why even need a healer. It would be so broken it would make WAR look like a baby.

It works in single player games because you have control over everything, it will not work in multiplayer because you cannot control everything. That is the core issue and unless that is sorted out, it will not work.