Straw Hat Pirates vs. Blackbeard Pirates - Inevitable. I definitely think that the last hurdle to Luffy's dream will be fighting against Blackbeard on Raftel. No clue if anything will happen between the two during the time, but it is very likely to happen. Of course it also sets up opportunities to Usopp vs. Van Auger fights, etc. Blackbeard Pirates are generally not well known as of yet in the one piece world, but they're sort of the opposite parallel to straw hats and like the SH crew they'll probably get more and more well known on the new world that they'll become a major player just like the SH crew where everyone knows who they are. It'll just be awesome to see these two groups who lead the new era clash. Similar to Whitebeard, Garp, Sengoku, and Roger who used to fight against each other all of the time, or Mihawk vs. Shanks.
Mihawk vs. Zoro - Another inevitable fight that will happen later on. We'll really see Zoro's progression and I feel he will achieve his dream and through determination eventually cut down mihawk. Another good idea would be to see them meet up sooner than that where they have a second fight, this time Zoro makes a huge wound on mihawk that becomes a noticable scar, proving that while he hasn't defeated Mihawk, he's strong enough to do some serious damage to the so-called worlds greatest swordsman, putting Zoro right up there in terms of skill.
Doflamingo vs. anybody - His power is very interesting and would make a pretty cool fight I think, and it'd be a good opportunity to reveal exactly how he received such a high bounty because people in One Piece don't get 340 million beli bounties for nothing, he must've done something major. I mean luffy's only 300 million and he's become one the biggest threats against the world government, destroyed enies lobby, and taken out 2 of the shuchibukai already, so anyone with a higher bounty than his right now must have done something huge.
Aokiji vs. Luffy rematch - It needs to happen. Luffy started the gear thing after realizing there were people like Aokiji out there, so it's one of those fights that he has to do otherwise he can never call himself the pirate king if he couldn't take on the strongest member of the marines.
it'd be cool to see the SH and RH crews go at it. I can't see it happening since they have nothing to fight over, unless maybe they have a little competition or something with each other, with shanks testing luffy to see how strong he's become and if he has what it takes to become the next pirate king, like to motivate him towards his dream, etc, especially because since the beginning Luffy's promised that he'd put together a greater crew than shanks.
) Vs. Magellan 
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