@Koliber:
Weird, neither of your examples got an ounce of frustration out of me. Well, Doflamingo turned out to be a disappointment, but that was mainly due to Oda deciding to use Dressrosa as a platform to kickstart the rest of the manga. And to the contrary, I believe these little bits with Blackbeard greatly build up his encounter with Luffy (and before the time skip they were a great build up for his fight with Ace and Marineford). And that's the kind of thing I'd like to see. I don't need to see Usopp reminiscing about Van Augur each arc, but having them actually meet pre-time skip or early post-time-skip and, who knows, maybe come to respect each other skills or start a relationship of any sorts would greatly help their future duel.
I agree about not having a problem with long-term character buildup like that at all. Just makes the payoff all the more special to me. Or disappointing in the case of Doflamingo, but that all depends on the end product rather than all of those years of waiting. I get not having the Blackbeard Pirates too hyped up to make other antagonists look weak or them getting beaten up too much on the opposite end of the spectrum. But I definitely think we could get more of a balance of relevance than what we've gotten. If this worked for Blackbeard, then I don't see why all ten other members have to be treated similarly if they do get some buildup past Chapter 1000 (goddamn, that's late as hell if they first showed up in Chapter 223 or so).
I never read Inuyasha, but I don't thin having one main antagonist for the whole series is necessarily bad. To have another example from a battle shounen, dethroning Orochimaru as main antagonist is something I still see as a mistake (though it has a bit more to do with Kishi's inability to handle him afterwards).
I don't actually have a problem with Orochimaru being replaced. At least on paper. Mainly because he never saw Naruto as a real threat. It was clear that he had more antagonism going on with Sasuke and the rest of the Sannin than he'd ever do with the fox boy loser who kept screaming about not liking his best friend get used like a plaything. I much prefer the final antagonist of a long running shonen series at least give the slightest damn about the main hero rather than constantly underestimating them as a sideshow nuisance to focus on bigger fish to fry. So a relationship like that being the final main conflict of the series would have been boring as all hell.
Of course, the way he went out was still terrible. As is the rest of the manga after his exit, barring a few concepts and characters. If this manga was written good, an ideal final antagonist would basically be a grounded, no nonsense ninja doing dirty jobs and deception for a selfish delusion claiming to be for the sake of the people. An antithesis to what Naruto wants to be as Hokage so that Naruto actually has an interesting relationship with the final antagonist. To determine what the true ideal for the life of a ninja should be like.
So… basically Danzo. You know, that ANBU root leader with the bandages over his eye and an arm full of Sharingan? Yeah, Danzo should have been the main antagonist. I know that suggestion seems random as hell, and I'm not saying he doesn't need to be altered in any way to fit that kind of role better. But like how Blackbeard's the most pirate-y pirate, Danzo's the most ninja-y ninja. Secretive, backstabbing, homicidal, cowardly, arrogant, manipulative, he's got everything an evil ninja leader needs.
He's got beef with Naruto because he forces his way into the seat of Hokage to do all sorts of sleazy totalitarian tactics (before all of that got cut so disappointing short). He's got beef with Sasuke because of all the Uchiha/Sharingan nonsense. He's got beef with everybody because all of the politics and manipulation from the shadows over the years. He used to be the Third Hokage's teammate and struggles with feeling like a coward, so you could work in a subtle Naruto/Sasuke opposite parallel since Kishimoto freaking loved doing that for every big villain since Pain. And best of all, he had good moral ambiguity to his actions. Where it could be debated that his actions are heroic in a consequentialist "ends justify the means" fashion.
It all could've worked in the hands of a better writer. The Hidden Leaf rising up against a corrupt Hokage (and possibly ninja world leader if kept rising up in power) would've been way better than all of that third ninja world war supernatural moon goddess giant chakra kaiju battle Michael Bay explosion nonsense. Or wait, you could even still have the war. But instead of the cheesy teamup between all the villages, Danzo forces the Hidden Leaf to fight against enemies regardless of moral alignment. That could have also been very interesting. Anything could've been more interesting than what we got.
I mean, sure, the other obvious options are Sasuke, Obito, or Madara. But... they're Sasuke, Obito, and Madara lol. Sorry for going off-topic, just... Naruto gets me thinking whenever somebody brings it up. Of what could been.