Omni as the 8th element, Lunar, Nodkrai
Columbina gives anyone in the party the ability to use Lunar reactions with her abilities even if they are not from Nodkrai
Lunar reactions resonate with most of the special reactions, perhaps more elemental reactions in the future (Pyro, Cryo and Anemo)
8 Dice, 7 elements plus 1 universal element
Omni Element can be used with any characters/elements
Paimon also has an "Omni Element" symbol on her chest, perhaps Paimon has something related
Lunar reactions resonate with most of the special reactions, perhaps more elemental reactions in the future (Pyro, Cryo and Anemo)
8 Dice, 7 elements plus 1 universal element
Omni Element can be used with any characters/elements
Paimon also has an "Omni Element" symbol on her chest, perhaps Paimon has something related
Commentaires
Theories without sense are more likely to be easily disproven, though.
As theories go, this one isn't completely without merit. Some theories suggest that Nibelung is the sun to the Moon Maiden's moons. His power - his "sunlight" - is what was divided into the seven elements. In real life, moonlight is reflected sunlight - and in-game Kuuvakhi could be a reflection of Nibelung's sun power. So, Nibelung's power dimmed, rather than refracted. This diluted power could be considered an "eighth element".
I don't personally think this is the angle they plan to go with this, but as a theory it's not totally ridiculous.
That's why I'd say Paimon having the Omni symbol heavily relate her to Celestia (Phanes fragment or regression theory etc...)