Would this come with any changes to AD beyond swapping it to a CD-less MP spender? If not, I suspect that Flood and Drain would cause one or the other among them to feel like extra baggage.
I could see Drain becoming an at-cost option for sustain, inflicting its enemies struck with a debuff that then causes you to heal for a portion of damage dealt to them and allowing them some synergy and Drain greater ST value, but if Drain remains a simple damage + direct heal per enemy struck, then you're just looking at the likes of 160p with no healing vs. 100p with some healing. The difference would feel all to small, I suspect.
That much is simple.
Sustain actions (whatever keeps you alive for longer without relying on denying your enemies their uptime) breaks down into four types:
- Increases maximum and current eHP, scales primarily only with players' stats.
- Increases maximum and current eHP, scales also with incoming damage.
- Increases current eHP only, scales primarily only with players' stats.
- Increases current eHP only, scales also with incoming damage.
Whatever scales mostly off of one's own stats has to be comparatively overtuned for low-incoming-damage situations to be able to compete in high-incoming-damage situations. And, all else being equal, whatever also increases maximum eHP is stronger than whatever does not.
The Blackest Night, along with any other %HP or potency-based shields or heals, scales just with your own stats, not incoming damage. And, of course, it increases maximum eHP (since it's a barrier, not just a heal); so long as that shield is fully consumed within its duration, it faces no disadvantage from not being permanent.
As such, if you were to amp up those strengths high enough to be competitive in content with the highest amounts of incoming damage, building around what DRK has done most uniquely since Stormblood while making its total sustain competitive with PLD or WAR... would actually put it ahead of either of them, reaching a new level of overpoweredness / unkillability.
As such, many are willing to take a bit more of a mix of types 2, 3, and 4 to keep it relatively balanced across the full slew of content. You may have noticed the overlap with those who want Bloodwhetting nerfed in dungeons (such as by having it heal off of damage dealt, rather than getting the full single-target value from each and every enemy struck).