I explained before that D. People are actually same as Punk Hazard Children, with Elbaph being our New Punk Hazard, and guess what? Now its confirmed Lily indeed lived there, clearly after her disappearance, so my theory still stands until proven otherwise,
While Sea Kings were created to be Sea God's Smile Army by some Scientist, who did PH Children resemble most? Oversized humans with legendary strength who go full destroyer mode if they dont get their sweets? Big Mom ofc, who was Kaidou's Equal and 2nd Villain and basically Monster He unleashed on Alliance,
So D. People are those experimented on in an attempt to turn one of them into a Monster who is as strong as Sea God, basically Mother Flame equivalent of beasts (similar to one Elbaph Children drawn, I mean the Children focus in this Arc is insane, its obvious they have an insane Lore)
This is why D. Is inherited by blood, not will, its a Name, and will of D. simply refers to their Fate, as their luck is because they are important for this Final Villain, one of them (as Vegapunk hinted, who btw is a Scientist) is gonna become that awaited Monster, Sea God's Big Mom, and whos better than Blackbeard?
Blackbeard is True D. He is the one CDs tell their children behave or He will come and eat you, and seeing how Blackbeard doesnt sleep + it should be a Sea Monster, related to octopus, devil, dreams, darkness ... etc and my best guess is Cthulhu, a Monster who can take on the entire World Alliance, at least before MCs Power-Up.