I still think Blackbeard is a strong possibility. Loki's got a Legendary Devil Fruit. Teach enjoys stealing Devil Fruits. It's a match made in heaven. The only real problem is that Blackbeard seems focused on Mariejois, right now. But, he's split his party before. I wouldn't be surprised if a few Blackbeard Pirates show up on Elbaph, even if Blackbeard himself keeps his eyes on Marejois.
Shanks and the Redhaired Pirates would also make sense to me. One, we've already seen Shanks has ties to this island. And two, it would be a good chance to have a "rematch" against Blackbeard if they show up. And third and finally, now that it looks like we might actually be getting "Pranks" in the manga, it would kind of make sense for Shanks to be here to fight his evil twin.
Cross Guild I also feel is a non-zero possibility. Mihawk might want to fight "Pranks". And I could actually see Blackbeard straight up hiring Cross Guild to join his side in the upcoming final war. Cross Guild WAS originally formed to be a business. It would actually kind of make sense for Buggy to agree to hire out his forces like mercenaries. Plus, if he joins up with Blackbeard, that means Buggy's probably staying safe until after the Final War is over, at least.
Having those three ALL show up would make this arc kind of packed. But, I could see scenarios where each of them show up.
The others, I feel, are far less likely. The Revolutionaries are focused on Mariejois right now. And unlike Blackbeard, Dragon doesn't usually split up his forces. And the Marines have basically been ordered to leave Elbaph alone for now by the Gorosei. I guess because the Gods Knights are taking care of the situation.
Honestly, I'm not sure if many more factions are actually going to show up or not. The Gods Knights might just end up giving this entire island the "Red Wedding" treatment. They might be the only surprise faction to show up. But, I could see those first 3 showing up, at least for part of the arc.
