@Smokeball
Off-topic, but I'd like to dispute your claim, specifically this one.
likewise for Shounen manga. That's why the more interesting manga would be in Seinen since it is technically gray zone for manga.
TL;DR Don't focus on demographics and tropes, rather focus on execution.
Seinen can also fall for the same tropes or follow a formula, case and point the "fantasy revenge" formula where the MC has been betrayed by the people they saved, goes crazy and dies, and then revived through bullshit, and then decides to go on a path of revenge. Here are 3 examples with the Seinen tag.
https://mangadex.org/title/34404/fukushuu-o-koinegau-saikyou-yuusha-wa-yami-no-chikara-de-senmetsu-musou-suru
https://mangadex.org/title/21758/kaifuku-jutsushi-no-yarinaoshi
https://mangadex.org/title/53235/my-lover-was-stolen-and-i-was-kicked-out-of-the-hero-s-party-but-i-awakened-to-the-ex-skill-fixed-damage-and-became-invincible-now-let-s-begin-some-revenge
If you read the summary, you'll notice they follow the same specific beginning with the same tropes.
Is that a bad thing? Absolutely not, It's actually fine. In fact I still follow kaifuku jutsushi because I got attached with the protag and want to follow his crazy and depraved revenge story. (can't say the same for fukushuu tho).
So what's my point? I'm saying is you shouldn't look at demographics and tropes, but instead look in to execution. Here is another example.
https://mangadex.org/title/22529/tokyo-revengers
https://mangadex.org/title/20316/rokudou-no-onna-tachi
These two SHOUNEN manga (though i argue tokyo revengers fit Seinen more) have many similarities. They both focus on delinquents; they both have protagonists who are utterly useless in a fight but has incredible endurance; both started out as losers but through extraordinary circumstances is given a new chance to change their lives; both protagonists builds up a group of friends through not only their extraordinary circumstances but also willpower and personality; both have an overpowered ally who is often used as a deus ex-machina to end fights when the protag has reached an epiphany or proven himself, but also becomes a looming threat of the series; and both are excellent manga that i highly reccomend,
and both experienced a huge decline in writing quality, common in long running manga.
Of course both have their differences (one is a harem/battle manga while the other is a battle manga) what made them great are the execution of their tropes.
Takemichi (of Tokyo Revengers) was a coward and failure, however when he found out about his highschool sweetheart dying in a freak accident, he suddenly went back in time. After understanding his newfound powers and meeting with an old friend, he became determined to save his highschool sweetheart and later newfound friends from a grim future. You see him grow from a coward to a loyal and unyielding young man. He may be useless in a fight, but he grew to inspire people around him to become better people, believing in their good nature and overcoming their problems.
Similarly Rokudou and his 2 best friends were outcasts in his delinquent school, wishing for a happy high school life. One day he received a special charm that makes delinquent girls automatically fall in love with him. Including the baddest bitch in
Town The Province Japan, Himawari Ranna. His magic made the unwieldy and bloodthirsty woman in to a love struck maiden who follows his every command alongside showing him affection to the extreme. However rather than using her to take over the world, Rokudou chooses to reform Ranna, at first out of concern for his own safety, but later because Rokudou genuinely cares for Ranna. Along the way he will also meet other bad girls that falls for him but then is helped by him towards the right path, he also befriends several male rivals even turning his bully in to one of his closest friends. Slowly he shapes his desired high school life not through the powers lent to him, but through his own efforts by befriending and guiding his fellow schoolmates. Rokudou grew from a coward to a young man that desires to love and be loved to the point that he even learns how to fight himself (best arc).
Another Shounen manga I HIGHLY reccomend is Mairimashita Iruma-kun (https://mangadex.org/title/21063/mairimashita-iruma-kun). This amazing manga follows the typical shounen beats. However, it follows them WONDERFULLY, in fact it can be argued that this manga follows the Hero's journey formula those snooty youtube essayists keeps talking about, but I'm getting ahead of myself. Iruma starts off as this unwilling human who was sold by his parents to a top ranking demon. This demon demands 1 thing from Iruma, become his grandson. Iruma has no choice but to accept, then he's sent off to demon highschool. There he makes friends through his own human understanding and past experiences making new friends along the way. The turning point is when Iruma must raise his own rank through demon dodge ball, there he follows old school shounen trope of a training montage. However what raises raises this typical shounen arc is how the manga uses this opportunity to show off one of Iruma's compelling personalities, his DETERMINATION, The manga shows Iruma that he was unwilling to give up, determined to master playing dodge ball even impressing his trainer who was apathetic towards Iruma. In the end, Iruma defeated his demon classmates even though they are much stronger and have magic and was awarded a rank up. Seeing his rise in rank inspired something within him, an unspeakable happiness overflowing from his chest as his friends congratulating him over his hard work and determination. This is his first time put in effort in something he wants. the first time he invested blood, sweat, and tears in to a goal he desperately want to achieve. The first time he gained a victory he EARNED, and it felt SO GOOD. And from there on he finally found somewhere he belongs, something he wants and work towards, and people he want to see happy and improve. This manga has a lot of shounen tropes done right alongside small subversions for the sake of humor. As of writing this reply, the manga is 184 chapters in and I have yet to find a bad or disappointing arc. The manga is at it's heights however it's nowhere near an ending. Though I fear the inevitable, I really hope it stays strong until the end.
er... anyways aside from my fangasming of Iruma-kun (seriously go read it if you haven't). I don't think the problem is in demographics or tropes, but rather execution. I think a manga fails when it doesn't understand the tropes it's using, thus instead of using it for it's intended use or tries to do something clever with it, It just slaps it in and think it's enough for the readers when it's not. You can see this kind of thing in slime shiitara (and kumo desu ga) and RE:Monster, both use the same system however the former(s) succeeded while the later failed. I'm not gonna explain because this post is long enough.
Edit: Grammatical corrections.
