Imho, its Luffy. But this is coming from somone who believes Shanks' motives to be cynical. I ask the crowd: Shanks didnt make a move and visit them until he saw Luffy's bounty. Nothing was even occuring before this point. Not even Blackbeard had made his move yet.
Theres a big question of why the Gorosei even brought up Sun God Nika during Onigashima. There was no direct reason for them to do it until all of sudden caring about Luffy's fruit.
But again, maybe thats just me