The thing with Koby is that Oda already established him as a great swimmer, so a DF wouldn’t fit him. He would be stronger if he just continues doing what he’s doing and gets ACoC.
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However I think Kizaru dying is a decent possibility. I see Saturn killing him and pinning it on Luffy.
Rather than Koby, I think the DF would be better with Helmeppo. Helmeppo doesn’t have any big haki feats, so imo he’d really benefit from a DF.