@ZONBI:
What makes you think that Crocodile didn't train to achieve haki? Did you see Bobo train?
How about the fact that, instead of trying to become stronger, he was trying to find a shortcut through an Ancient Weapon? Similar to how Moriah tried to use Oars?
Do you even know how much of a NW veteran Crocodile is? How much time do you think he spent there? I think you're really overestimating Crocodile there.
Oh, and Sai and Boo have been mentioned to be always training, as they're the leaders of the Happou Navy. It seems logical to believe that they would train at Haki fighting, especially with their grandfather being a CoA expert. That doesn't mean they're stronger than Crocodile, but they actually went through the proper steps to achieve Haki.
That's more than I can say about Crocodile. He doesn't really show any remarkable skill outside his DF powers. Luffy needed 2 years of proper training and he's not even a Haki expert yet.
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@sabinis:
I would like if Crocodile would stay somehow rival of Luffy in a way and if 3rd mate of Strawhats is stronger than Crocodile.. I dont see that.
Croco was pretty strong guy before Timeskip, some 2 years of polishing in New World and he gets back to his best I would say.
He's most likely going to be overshadowed as his time as a main villain is over. And Luffy already has 8 other rivals.