Hi
@Wrobel - as you like to see numbers, I thought I would channel my inner
@Asayre and dust off Matlab for some analysis of Dragon Blood.
TL;DR is at the end.
The Issue
Dragon Blood is not fit for purpose - the scaling system used to calculate the heal is inferior to most other heals in PvE and all other heals in PvP. The current system rewards only those who stack massively into health, and the lack of stat scaling makes it a dud for min-maxers.

The general trend of the heal can be seen in the above figure - it is clear that, for the average 18-25k health player, the heal received is disappointing even at low health. For a player with 20k health, the largest base PvE heal that they could ever receive is 6,667 - and to receive this they have to effectively have zero health remaining.
A Proposal
My interest was piqued by a point made by
@Joy_Division here.
The idea was to set a base heal for DB that scaled on max magicka and spell damage, as do most skills, but add a modifier based on your missing health.
After a bit of tweaking and experimenting, I tried the following formulae:

The base heal is defined similarly to other skills; the a, b & c values were just manipulated to give reasonable values - they may need adjusting.
The first term in the 'heal' equation is the low-health modifier - it is based around an exponential growth, but modified to ensure that it doesn't scale too quickly, increasing significantly at low health.
A plot of this heal is shown below - the three planes correspond to three increasing magicka/SD combinations.

The bias towards health-stacking has been reduced to a less significant increase with higher health: the focus is that you get a bigger heal when you have lower health.
For a player with 20k health, the PvE heals would be:

These values are not perfect and the a, b, c and modifier values would need adjusting to optimise and balance the skill. Obviously the heals are half this value in PvP.
Issues for stamDK
The fact that the heal would scale from maximum magicka and SD
should prevent this from being an unneeded buff to stamDK; for a 12,000 magicka, 1800SD player the largest possible heal is 4028 at zero health. Again, this could be tweaked by adjusting the a/b/c values. For GDB I'm in two minds as to the best change - adjust the heal component to the new formula, or just leave it as it is? AFAIK stamDKs who use either morph of DB use it for the buff rather than the heal - but I could be wrong.
Other Comments & 'Edge-Cases'
The issue with the health-stacking magicka DK should be lessened by the changed formula; higher max health gives slightly larger heals between 99-20% current health, but when the big heals kick in at low health they are about equal across the board.
True hybrid builds who have reasonable magicka/SD could get a fair heal from an adjusted DB. Good for them - I don't see this as an issue as they are hurt in so many other ways by playing a hybrid.
I'm still in two minds as to whether a changed DB should be able to crit: if you're at low health then DB will give a solid heal - a 1.5-2x crit multiplier could lead to ridiculous builds that consistently shoot back up to full health at the push of a button (don't bring up BoL!). In a similar vein but with a different conclusion - I see no reason why this shouldn't be affected by major mending. Builds with access to major mending are already using it to get obscene burst heals, so I don't really see much of an imbalance.
Conclusion and Comparisons
The idea here is not to make DB an OP skill - if that is what this suggestion does with the current values then they need adjusting. This isn't supposed to be a DK Breath of Life; it's an attempt to make an underused and outmatched skill somewhat viable. In PvP it will not give very large heals, but hopefully they will be enough to at least give people the option of getting good usage ou of DB.
TL;DR
Change the DB heal to give a base value scaled off max magicka and SD, increased based on your missing health. Probably shouldn't be able to crit to avoid ridiculous heals.
I suck at getting images right on the forums - bear with me if they aren't in the right places!
PS4 EU - Panth141 | CP 630+
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