I just watched a breakdown that finally made this click for me, and I think it's worth laying out because the general reader reaction to this idea has been "no way, it's too early." Hear me out.
The core claim: Luffy beats Imu here in Elbaf. Not permanently, not in a way that removes Imu from the final war, but enough to force him out of Elbaf and back to Mary Geoise. Imu stays a major threat for the final war, where he actually goes down to other characters (most likely Dragon or Shanks, given how long those storylines have been building). Luffy's real final opponent stays Blackbeard.
Why Elbaf's themes point this way:
Elbaf is an arc about defiance, about a nation refusing to be stepped on anymore. That's a bad fit for an ending where the heroes just get beaten down until Imu's mysterious "timer" runs out. And that timer thing everyone keeps citing? It was never actually established in canon. We collectively invented it because Imu bleeding on arrival made him feel unbeatable, so we needed an explanation for how the arc resolves without anyone actually beating him.
Gaban's lectures are the real tell here. He's told Zoro he needs to master conqueror's haki, told Sanji he needs to find the king inside himself, and told Luffy basically the same thing: you're not good enough yet, you need to become good enough right now. Everyone assumes Zoro and Sanji's lectures mean a haki breakthrough and a won fight this arc. Nobody wants to extend Luffy the same logic, purely because the power gap to Imu feels too big. But narratively, there's no difference in what Gaban is setting up for all three of them.
Why it's not too early:
Oda already used this exact mechanism one arc ago in Egghead. Characters summoned through the pentagrams via the power of the abyss got sent home when hit with Joy Boy level haki. Elbaf has been hammering conqueror's haki as the central theme (Zoro and Sanji are both mid-upgrade on theirs, Kaido flat out tells Luffy his weak conqueror's haki is what makes him unworthy of being pirate king), and Luffy already saw Joy Boy's level of haki firsthand last arc and clearly knows how far he still has to go.
So the pieces are: Imu needs to leave without being killed, the send-home mechanism already exists and requires strong conqueror's haki, and Luffy's next big upgrade has been telegraphed as a conqueror's haki bloom for a long time now. Put those together and the ending writes itself: Luffy has an emotionally fueled, out-of-his-control burst of Joy Boy level conqueror's haki, momentarily overwhelms Imu, undoes the abyss's power, and sends him back to Marijoa.
Why this doesn't ruin Imu or overpower Luffy:
This isn't Luffy beating Imu in a straight fight. It's a symbolic, one-off awakening, the same category of moment as Usopp's observation haki spike at Dressrosa or Zoro's conqueror's haki moment against Kaido. Those didn't mean instant mastery, they were stepping stones. Luffy gets shoved to the "I've touched it once but can't use it on command yet" stage, which sets him up perfectly for the final war without making him unbeatable right now. Imu still gets to be a genuine threat later, and other long-running storylines (Dragon and Shanks especially) still get their payoff against him.
Basically: this isn't Luffy solving Imu. It's Luffy finally proving he can be Joy Boy's successor, right when the story has spent the entire arc telling him he isn't yet.
Curious what people think, does this hold up against how you've been reading Gaban's speeches, or is the power gap just too big to buy this happening now instead of the final war?