Luffy's subordinate? that's lame
Luffy is too much of a bitch to ever kill anyone, he rather befriend them or let them live Batman style, no matter what they do yet he acts mad about Kaido starving and enslaving people for decades.
If Kaido doesn't die, he'll probs end up becoming a pseudo ally like Crocodile in future and helping Luffy lmfao. I bet even Doflamingo will get a similar role and will get a more successful future too since Oda. I doubt he's gonna stay in Impel Down somehow nor die.
Buggy, Crocodile, Wapol and such have only succeeded AFTER Luffy beat them ironically, how does that work? He may have saved those towns and countries but those villains are getting more popular, powerful, successful etc ironically.
Oda really went meta with making Luffy a cartoon because that's what he is. A joke, a cartoon. More so than he should be in the literal sense.