Seems reasonable most of the planets minus Earth would be the Elders, with the last three being the Ancient Weapons, and Nika is the sun, obviously. But I would cast Imu as the moon - given their big space laser and the amount of ancient history tied into it and the Lunarians. Plus the obvious angle of the moon being an ultimate opponent of the sun. I don't know that Earth necessarily needs a representitive, given that it's both the battlefield and the thing being fought over. Blackbeard representing a black hole I think could be a fun extension of the space theme.
I'm going to need more info in the story before I'm willing to believe there's anything more to all of these than a nice collection of themed names though. Like, we're not going to find the ending of the story in the real world myths the celestial bodies were named for, it's just some fun surface-level stuff with at most light symbolism. Oda didn't give Doflamingo the family name he has to send fans looking for the end of his arc in the literary Don Quixote, for example, it was basically all for the reference.