More like Thriller Bark redux. It's an exorcism arc!
Giant big bad with focus on necromancy/evil magic. Everyone needs to team up to beat them, but has to beat their individual fights first.
They CANNOT defeat Imu here. The best case scenario they have is knocking him back to Marijoa. After, they need Shuri's help to destroy the Magic Circles so Imu and his troops can't just spawn back in.
While it's possible there's another method, I think Egghead already showed us the solution: yeet Imu as far away from the Summoning Circle as possible.
We know doing physical damage to the Gorosei/God's Knights is worthless without enough Haki to counter them. It's possible Nika Bonney has CoC (still wondering about that), but, Sanji and Franky don't here, and I'm not sure how much CoC Luffy can transfer through getting punched in Balloon form.
IMO this page proves that if you knock a Gorosei far enough away from the Magic Circle, they return to Marijoa. Since Imu was also summoned here via circle, I think it's the same mechanic.
They need to YEET Imu out of Elbaph so far it breaks the connection and he returns to the Holy Land.
Wano Act 4 will be the first War arc since Marineford, with every Yonkou, and Admiral, showing up to fight over Pluton, along with Dragon, the Whitebeard/Roger Remnants, and others. BUT, unlike Marineford that was decidedly one side vs another, the Whitebeard Pirates and allies vs the Marines and Allies, Wano Act 4 will be different.
The War for Pluton, Wano Act 4 will be a battle royale where everyone is going for Pluton, and alliances/betrayals will happen all over the place.
Which is why I want it so bad. A battle royale where the Yonkou finally fight the Admirals. YESSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
I do think Kuzan will try to kidnap Robin when he shows up on Elbaph, though. But towards the end of the fighting to be sneaky.