Hey, with to your explanation, and without it, when we see Milim Nava and Velzard, who are still in sealed form, I tell myself that the true hybrids also have a complete form, which could be based on the following beings:
Yang - A form with rock-like scales and boosted abilities mixing space and earth (the opposing elements of Veldora and Dagruel) based on the Behemoth (a mythical beast representing the earth). This would be the only one not to have dragon wings, as it is purely terrestrial, but it can float thanks to the gravity manipulation granted by the land attribute.
Noah - A form using powers linked to water in its four states of matter (thanks to Rimuru and his physics magic, it could generate plasma water) but having black scales from which darkness escapes. These dragon wings would be at the pelvis level to recall the demons of lust (Luminous having the ultimate skill "Asmodeus - Lord of Lust") and would be black with an icy membrane (as Lilith from Black Clover) and a pointed tail and a double pair of horns. They would be based on the Leviathan (a sea dragon representing the sea; a logical consequence since his mother has the ultimate skill "Leviathan - Lord of Envy").
Velgrynd's son - I see them very similar to Acnologia (in appearance more, but in reality scales), in red and in three pairs (six wings as Seraphim) and radiating firelight (like Michael's Flaming Sword against the dragon Lucifer), with a fiery halo on its head and based on the Ziz (a bird representing the sky).
Silas - I imagine her form will be closer to her father than Milim, having blue scales as well as her dragon tail and wings, with temporal powers probably equal to/superior to "the one who sealed Veldora". He wouldn't actually have any (maybe not even physical changes) because he's still in his true form and still superior to other true hybrids (like God who can control Behemoth, Leviathan and Ziz). I also have a question: What Japanese voices would you give to our true hybrids?
