DRK changes that would make it really fun to play

[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.