Blackbeard will use Shinobu to age up the seraphim and Catarina Devon to control them.

We know that Blackbeard received info from Caribou about the whereabouts of two of the ancient weapons: Poseidon and Pluton. It's not a coincidence that the Blackbeard Pirates were last seen at the bottom of the Red Line above Fishman Island. Then we learn that Fishman Island was under attack but is safe now.

Since the Blackbeard Pirates failed to retrieve Shirahoshi, I imagine they'll make their way to Wano next. Blackbeard uses his Tremor Tremor fruit to destroy the walls around Wano at some point, freeing Pluton prematurely (ahead of Monosuke's schedule).

Of course Monosuke, Yamato, and the scabbards won't just let him run free and will resist. At some point he encounters Shinobu and witnesses her power (likely after seeing how Monosuke has grown).

Being the opportunist he is, Blackbeard will see the value in her power and decide to either take Shinobu or her fruit.

He leaves Wano in the aftermath of releasing Pluton, his actions are seen and taken as a declaration of war by the Strawhats (considering Wano is Luffy's territory).

Skip to the final war. Many believe Imu is the final villain, but I (and many others tbh) believe Blackbeard is more likely to take the spot as the final villain.

Imo, Blackbeard either contributes in the big battle to take down Imu, or, is nowhere to be found during the battle against Imu. Then after Imu is defeated the Gorosei and the Holy Knights will lose the benefits that come with their covenant with him. Those who have lived beyond their natural lifespan (particularly the Gorosei) will experience the same fate Saturn did when his power was removed by Imu on Egghead Island and will die. Those like Garling who only recently received immortality will simply become mortal (along with losing the regeneration, power boost, etc).

Seeing as the Gorosei have all been alive for centuries and will be dead, then this is perfect for Blackbeard, who I believe will swoop in and use Catarina Devon to change into Saturn.

Remember on Egghead her mission was to touch Saturn. Given her abilities, it would allow her to take his shape and identity which would make her the highest authority to control the seraphim.

I don't believe Blackbeard will control each seraphim. I believe he will lose/get rid of the members that were present during the Hachinosu raid and replace them with certain ones. However, this part of my theory isn't mine to share so go check out Mr.Morj on YouTube (he's great!)

Regardless of which seraphim he acquires, I believe he will either force Shinobu or give a member of his crew her fruit so that she/they can age the seraphim to their maturity just as Momonosuke was.

Remember that despite what they've displayed, the seraphim were explicitly stated to only be in their child state. Once matured, I believe they'll surpass their living counterparts otherwise what's the point of Oda putting them and that fact into the story.

This would make the final battle of the Strawhats vs the Blackbeard pirates line up well.

Given that Conqueror's Haki is hereditary, I see no reason why those such as S-Hawk, S-Snake, and S-Flamingo shouldn't possess CoC in their adult forms just as their originators do. And this late in the story, ACoc is the name of the game.

Naturally, for you Zoro fans, this means that he will have a final opponent who is definitively stronger than Mihawk. (And by that point he should have already bested Mihawk anyway).

Anyway, this is how I see things play out concerning Blackbeard's placement in the final war. Do you agree or disagree?