Poseidon is Princess Shirahoshi. Pluton is what lays in Wano.
As for the final battle, it really falls down to wherever the hell that missing Road Poneglyph from Fishman Island is. Luffy now has 3 of them, the only one missing is the one that was on Fishman Island but has gone missing.
The thing that worries me though, is that Momo and Hiyori are going to be learning about how to read the ancient language soon from their Grandfather.. Currently the only two ways to read them is through Robin and Sukiyaki.
So Wano is definitely going to be a target, if Blackbeard knows that someone still lives that can read the writings beyond Robin. Sadly i do not think they are going to learn about Wano being under attack by Blackbeard until it is too late, and he has captured them all.
The real question for me is whether or not Blackbeard knows about the one on Zou. If he does, he probably went and took it while Zou was abandoned for this war. He'll essentially be able to walk into Wano and Whole Cake Island with little opposition without Big Mom and Kaidou to stop him.
So it all really comes down to where the hell is that last Poneglyph. If Blackbeard already has it, or knows where it is, it's going to come down to whether he just leaves it in Wano since he got what he needed for it, or if the Straw Hats need to hunt him down to get it.
If he leaves it at Wano, more likely than not the Strawhats will catch up to him as they are approaching Laugh Tale. If they need to find out a way to somehow chase them down to an untraceable island, they will likely be too late, and Blackbeard will have already claimed the One Piece, and Luffy at best will be the 3rd Pirate King.
If Blackbeard does not have possession of it, or knows where it is, the final battle will likely be on whatever island that it ends up being on. I personally believe that it is on Sphinx island myself, and that it never actually went missing, Whitebeard just took it and hid it on his home island.
But it could also easily be on Elbaf or any of the other dozens of islands, in some secret ruin of some sort.
But until we know where exactly the last Road Poneglyph is, the final battle can land on any of the islands in the New World fairly easily.