So knowing now that Kaido can literally move entire islands, what are the odds that he played a part in the disappearance of God Valley? There’s still much of his character/past shrouded in mystery and knowing that the folks involved in the God Valley incident don’t like talking about it, I can see him keeping that to himself if it were the case.
However the support for the idea that Onigashima could be the missing God Valley moved by Kaido himself so far looks quite weak. Kaido only came to Wano after Orochi laid the ground work gathering gold and building factories to lure him in. Oden called it an empty little island as he knew it all his Wano life. Kaido says he only recently settled it and built a mansion there.
Izo says the same thing here. He has not been in contact with Wano for 30 years. It was a poorly accessible, empty island with no name to the Wanonese and Kaido named it sometime after Oden’s death. We have no reason to believe Marco knows anything about Wano, knows anything about God Valley. If Whitebeard told him about his shameful past then Izo would likely have this knowledge as well.
The second point is why would Kaido preserve the death place of Rocks D. Xebec? He glorifies Roger, Whitebeard and Oden’s death. He never says anything about Rocks. He hates their former members Big Mom and Whitebeard. Is Rocks alive and Kaido is waiting for him on this island? We don’t have a clue if he admired Rocks or thought he was a powerful fool. Why not take Hachinosu, which is also a giant rocky skull and the birth place of the Rocks crew? Whitebeard might have gladly sold it to Kaido considering how distasteful his memories are of Rocks.
The last point is that God Valley was where some Celestial Dragons were living away from Reverie. For the nobles to live inside a cavern in a grotesque rocky skull would be some bizarre taste. To leave no signs of their civilization behind is just really out of place. If Kaido wiped it all before he moved the island, you would at least see signs of destruction and not just barren rock on the outside. I get the feeling this theory only became popular this week as the fossilized head revealed to be missing the rest of its body shut down any ideas about reanimating an Ancient Giant.
*Theory by Lampyris