Just noticed something, adding to all my theories:
Luffy fought Kaido 3 times after all.
The first one, he was imprisoned after, a total loss. / Like Kid lost to Shanks Crew the first time big and lost his arm.
Luffys second fight on the rooftop ended by him drowning in the sea / Kids second fight against Shanks ended him drowning at Elbaph.
Luffys third fight ended with him winning in his awakaned Form on the rooftop / Kids third fight is up to come at Elbaph.
but i still think the target wont be Shanks himself, its the guy who "killed" Victoria. Shamrock, which took Victoria / Gunko with him.