General & Others The final war will be a giant battle royale

The strawhats won't fight the the BB pirates before they fight the world government or vice versa. The strawhats, BB pirates, WG, navy, RHPs, and the Revolutionary army will all be involved in one giant battle.

Oda likes to parallel the arcs of pre and post time skip. Egghead arc has many parallels with Shaboady, one is the first arc of the Marineford saga and the other one kicks off the final saga. After a couple of short arcs, the marine ford saga culminated in one giant battle that involved the navy, warlords, the WB pirates, the BB pirates, Shanks, and the impel down prisoners. Every faction had their own agendas. The final battle will be the same, and Oda once stated that the final war will dwarf Marineford.

Oda's comments at Onepiece jump festa 2023:

"For the future of the manga, no way!! that person and that person are going to be fighting!! That's the type of battle you can expect, free for all battle royale".

A common misconception is that Whitebeard stated that when the Onepiece is found, the greatest war the world has ever seen will engulf the globe. Whitebeard never specifically stated that the Onepiece will be found before the final war, in fact, Oda mentioned in an SBS that the Onepiece reveal will be the final part of the series, or else the readers might lose motivation to continue reading after the big reveal.

So No, the BB pirates won't be defeated in laughtale before the final war against the WG. If you think that the villain who has been built up for 900 chapters, the antithesis of the MC who holds the power of darkness itself, who carries the will of Xebec the celestial dragon hunter, will become a minor arc stepping stone a la Wapol and Ceaser Clown, then that's borderline illiteracy. There's no way that BB falls before the accountant and the lawyer of the WG, and we don't have the time for 10 1vs1 battles between the strawhats and the BB pirates at laughtale with Oda repeatedly stating that he wants to finish the series as soon as possible.

Similarly, Shanks won't be offscreened by BB before the final battle, that's Oda's favorite man, and Akainu, the head of the strongest military force in the world who has a past with the MC, won't be devalued by having him get beat by a subordinate like Sabo. What will likely happen is that Luffy, BB, Imu, Shanks, Akainu, and Dragon all will represent their respective factions in the final war, with Luffy vs BB vs Imu being the final battle of the series. We might see something like that:

Luffy vs Akainu -> Luffy wins
BB vs Shanks -> BB wins
Imu vs Dragon -> Imu wins

Or we might see Akainu vs BB or Luffy vs Shanks, it's a free for all battle royale.

Similarly, CG's main goal is to find the Onepiece. If the strawhats find the Onepiece before Zoro fights Mihawk, then the fight will be meaningless because CG will have no reason to continue fighting anymore, it will be a personal fight between Zoro and Mihawk which defeats the whole reason Mihawk joined CG in the first place. But the final fights for Zoro will likely be Shiryu, Venus, and Mihawk, all at once, with him fighting Mihawk for the title of the WSS at the end. We might even see Mihawk clashing with Shanks and the other #1 with him representing CG in the power category.

The navy will most likely have internal fallouts. The story has repeatedly stressed that the navy is full of good marines who are against the WG. People like Smoker, Tashigi, Fujitora, and Sword members wont fight for the slave owners. And with the news of the crimes of the WG spreading between its members after the great cleansing, and seeing civilians hunting them down and offering them to CG because of hunger, Oda is setting up a civil war inside the navy head quarters, one faction represented by Fujitora, and the other is spearheaded by Greenbull.

In fact, the one thing Akainu has done since the beginning of the timeskip was questioning the five elders and the WG, and that says something. It is why Oda has introduced God's knights and the Gorosei as the main force of the WG in the final war, not because each one of them is a top tier stronger than BB and Akainu, but to separate the navy as an independent faction in the final war.

The admirals will have their own agendas. Some like GB will most like side wirh the WG (Prejudice breeds stability), others like Fujitora will certainly betray them. Akainu might have his own agenda and might start a military coup to take the power from the nobles, or he can stay as a government dog and fight for Imu. Kuzan and Kizaru are wild cards.

I think that after the strawhats win the final war, they get the final poneglyph, immediately find the Onepiece, celebrate with everyone, and we get an epilogue and that's it.

The only thing that is left is to speculate about the battlefield and the event that triggers the final battle. The holy land is the most likely answer. In Marineford it was Ace who started the war, it is possible that the great cleansing and Vivi's capture will cause the strawhats and the revos to attack the WG and the the other 3 emperors will follow them. Alternatively, it might have something to do with egghead incident or the grandfleet incident, it is not clear yet.