I agree about ToG. I still like it a lot, but it is not the great ride it promised me to be.
There are very few works who keep being interesting after a lot of content.
Some become incoherent, like claymore or (as a much more extreme case) bleach.
Some are always the same without anything fresh with premise who gets bland soon enough.
And for some, you simply stop caring about the characters at some point.
A manga who is more of a seinen but did a lot of things right for me is vinland saga.
The arcs always advance the plot in a substancial manner and there are no 'wasted characters' who just stick arround without doing anything.
It actually stands to its premises and has an interesting and believable way to show how the maincharacter came to his idealism.
The characters are (mostly) genuinely charming or interesting.
The arcs can differ greatly from each other, with some of them having almost no fighting involved.
Lots of those points are true for HxH too. Togashi puts a lot of effort into using his arcs to the fullest to advance the narrative.
I always like to use the skypea arc in one piece as a negative example for that:
It was a solid arc, but it focussed (like many one piece arcs) almost exclusively on itself. The backstory of the sky island was fleshed out very well, but, all in all, if this arc had not existed.....you would basically not have any difference in the one piece overall narrative (given that the stone of that island would have been located somewhere else, of couse, and their logue port had pointed them to that place instead).
Enel was a one off villain never to be seen again (except in the mini cover arcs), the same goes for all characters on that island.
Nothing substantial between the strawhats happened there (like usually, the relationships have basically stayed the same), so no changes in relationships or insight on relatiionships between them.
The only thing it introduced which was somewhat used throughout the manga were the dials, but you didnt need an arc for that.
A lot of manga only focus on the arc they are in with everything they got, and i think, on the long run, this is a mistake.
Even worse if the mangaka TRIES to let the arc have impact and at least keeps using some of the stuff introduced in it, even if it does not really add anything to the work.
In Tower of God for example, a comic i actually like a lot, there was a point at which SIU just started adding too many characters and kept them arround for no good reason. They were not really interesting, not by fun standards nor by plot standards, they were just there. And since he felt he should use them sometimes at least, they get their moments some time, but...it's not thrilling.
The only reason a character should get spotlight is when he is either interesting on his own or relevant to the plot.
Of both is not true, keeping him arround is a dead weight. EVEN if he might become important to the plot 100 chapters later.
Worst offender would probably be gantz, in which Oku himself didnt know what to do with the vampires anymore and just kept them arround at first for no good reason (and same goes for the espers really) and when he got tired of them, ignored them after he had hyped them up oreviously.
HxH luckily does not do that most of the time. Almost everything which is or was given spotlight has had some relevance.
A nen concept being introduced usually keeps being important (and not like in shamo, where ppl learn ki in one arc because its cool, and then forget it because its overpowered) and relatively consistent.
And the stuff which happens in arcs which is given spotlight even tho it is not important for the major plot is usually really interesting.
For example: Komugi playing with the king lead to something. Komugi was, of course, a plot device to hav meruem und netero have the one on one fight, but she was fleshed out to be much more than just that in the end.
If togashi had done that wrong, he could have ended up boring chapters with 2 people just playing a game, or, in worst case, explaining us the rules of the game even tho we CLEARLY didnt need those (like, sadly, often in ToG the rules of the games are not all tha timportant and it ends up being decided with fighting anyway).
