Yes, Imu never stood a chance to be Luffy's true final opponent.
Blackbeard should be Luffy's last big fight but not the last one.
Buggy is Luffy's first fight once they crew was formed and it will likely also be Luffy's last fight.
Imu and the Gorosei have cool Satanic/otherworldy vibes and so did Enel. So did Kaido. So did Big Mom.
But Teach does what none of them could: He wins while being human. Dirty. Flawed. Hungry. Weak. That's precisely why he's an anomaly amongst Oda's creation.
Luffy = Sun God – Freedom, Joy
Imu = KotW – Control, Fear
Blackbeard = The Abyss – Gravity, Oblivion
Now It's said, they're like Elbaph's three worlds.
Luffy wants to free the world.
Imu wants to rule the world.
Teach wants to end it.
"But Xebec wanted to rule the world not end it" People see Teach and just assume he’s just Xebec 2.0 which is farthest from truth.
Xebec wanted to become King of the World. Wanted to replace the Celestial Dragons and reign through domination over the system. He had a vision of global monarchy. Very likely also plans for post-conquest power.
Teach's ultimate goal is just power. He will consume the world like a blackhole, not rule it. It's quite literally what he subconsciously said in Marineford.
Teach used every faction from his very family, Warlords, Marines, Yonko and soon Nobles as a stepping stones, then broke it behind him. Xebec would've never agreed to serve the government or other factions. That’s less of aconquest and nore of an extraction and collapse.
Harley texts also implied that the "troubled shadow” that “remembers the promised day” and listens to the “sound of the fragmented Moon.” which could be Teach, would play a role in a 3rd world-ending event, that could be prevented for all we know.