I am confident it will be whoever goes after Cross Guild, Oda is probably itching to draw it. We will know then. I would say most likely option is currently Greenbull being beaten by Crocodile and Mihawk slapping Sengoku when GB/Sengoku go after them.
Crocodile definitely has enough of a natural advantage to offscreen Greenbull, and all we would need to catch is Mihawk giving Sengoku his proper respects before putting him down.
Notable, though, is how confident Sengoku seems. He is the guy after all who had one of Whitebeard’s captains betray him under his nose.
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At this point, the only high-top tier that Oda might not have a way to feature is Magellan, whose role ties him to keep watch in Impel Down, not take part in a full-scale Final War.
So if CG breaks down into ID in order to recruit Doffy and Weevil, like most theorize, there's a perfect chance for everyone to be scared shitless about Magellan and Mihawk being able to show off against him.
If not Magellan... Either an ex-Rocks that might still be wandering around (Silver Axe?) or a Seraphim. Can't see Sengoku going down to anyone but Blackbeard after the whole Garp situation.
Broooooo
The raid on Impel Down to breakout Doflamingo and finally introduce Weevil is a top tier possibility. Crocodile is exactly the kind of strategist that would do that. Breaking people out of Impel Down is the thing that gave them all their success. Now with a rejuvenated prison population and two monsters at least kept there, it is the perfect time to do it while the government is down an admiral and fighting on multiple fronts.
It is exactly the kind of thing we would see on a newspaper at the end of Elbaf. Magellan killed and Impel Down destroyed by Cross Guild.