interesting topic even if its far fetched
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let's see
1-king now way
2-shiryou also no way he is main villian
(but one thing here if kozan still at that time with teach then yes luffy could ask him)
3-benbeckman hmm luffy could ask him as well but he will not join .
4-kata and marco no
They are both at same situation lost their captains
5-killer hmm yep luffy could ask him but he will not join
6-law bear no he is not anymore luffy rival
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only kuzan who seems possible
cuz maybbe he want to know the true history