@Solgarde:
I think maybe that ultimately the ending isn't going to come down to "All of the world's systematic problems can be solved by punching this one singular person really, really hard enough times"
Thus far, it seems like each villain in the series has either explicitly or implicitly been motivated by some negative experience and often, if only in a twisted way, are just trying to either achieve something big and ambitious– or are honestly trying to make the world better for most people, even if it would hurt some people in the short term.
Luffy's most frequent antagonistic force have been... well, the people trying to stop those running around breaking the laws and causing chaos from disrupting the lives of the people who just want to get on with things and be peaceful, productive citizens rather than devoting their whole lives to learning to fight. And well-- often that order is corrupt or some nasty things are accepted in order to keep total chaos and massacre at bay, but... they are just trying to protect people.
So if the ultimate end is "oh, there is this one individual who is all bad without reason and everything wrong with the world all ties back to him and once Luffy beats him to death with his bare hands, everything in the world will be perfect" it feels like it would totally undermine some serious themes that have been there since the early stories.
Although I can't say I have as much respect for the later post-timeskip stories as I did for the earlier ones.
In Alabaster, the people were going to kill each other over what was a perceived injustice and while Luffy beating up Crocodile mattered-- the thing that stopped there being a big war was that luckily Luffy had on his side was someone loved and respected by both sides who could get them to stop.
In Dressarosa, it really was just one guy who was responsible for all of the death and destruction, and just by punching him really hard enough times Luffy was able to save people. Although Luffy certainly took his sweet time doing it and if there had been any realism in the series at all, 80+% of the people of Dressarosa would be dead by the time he actually decided to get around to using his ultimate technique that he could have used from the very beginning and win the fight. The little deus-ex-machina of just declaring that everyone, even the slowest citizen, even the citizens who may have been asleep, that everyone managed to escape the bird cage and squeeze into that little 10% of the nation that wasn't sliced to ribbons is among the dumbest of hand-waves to spare the main character from being a real monster that I have ever seen in a story.
But I still hold out hope that the maturity we saw in the earlier stories could still be there and there won't just be some "final villain" who by beating to death Luffy will save the whole world and everything will from that point on be perfect for everyone and everyone will forever be on their best behavior.
Yeah, we might have Imu. Or maybe Imu will be killed by Dragon and Dragon will take over the world and use its power in far more aggressive ways than the previous World Government would have. Or will have some ideological battle between Akainu representing ultimate relenting order and Blackbeard representing unhinged, irresponsible freedom and Luffy will need to overcome both.
But I just don't think that the ultimate test of Luffy's character is going to come down to a singular individual that he simply has to physically overcome.
Also, I notice that almost everyone on these boards thinks Blackbeard is the final test.
But... really, Blackbeard seems like the person Luffy will need to race with to get to Raftel or the One Piece first.
Yet if Luffy is going to destroy Fishman Island and deal with the Celestial Dragons and create the All Blue and such-- none of that is going to happen before he gets to Raftel and obtains whatever the "One Piece" is and learns the lost history of the world. That's all stuff that is going to happen after he gets there and gets those answers.
So I really don't see Blackbeard as the final opponent. Blackbeard is built as the opponent for Raftel arc. But if Blackbeard loses that race, then it feels like his potential significance in the story afterwards is inevitably greatly diminished.
I get that IMU is the leader of the World Government, who have been the overarching antagonists (alongside Pirates also, anyway), and that Akainu killed Ace and all.
But honestly…it would feel weird for me if the final antagonist of the pirate manga, isn't the pirate who's the evil version of the main character (with the D. and all), alongside his evil version of the main character's pirate crew, and he's even a big part of Ace being dead anyway considering he turned him in, and he's been mentioned constantly throughout the story since Drum, even before his initial apperance. And considering that he's going to kill Shanks and all, he can make up for that whole "Luffy holding a bigger grudge against Akainu" thing.
Although...Dragon I don't understand. Dragon's been a pretty good character up until this point. Heroic, even. At least, from the little he's done. And his right hand man, Sabo seems very much not evil.