I just took a look at Koe no Katachi out of curiosity and I'm quite interested, though I'm only about 1/6 through it. It got me thinking of something I'd been wanting to see for a while: a story where a guy is bullied and treated with a great deal of scorn and abuse, who manages to somehow find a way to make the bully(ies) see him in a different light, gaining their respect, hopefully a friendship or desire for friendship, or even romantic feelings if it's a person of the opposite gender. I guess this is a product of a less-than-pleasant childhood (few of us have escaped from bullying entirely and I certainly have known my fair share of harassers), but anytime I see a bully I want to see them truly regret their actions, truly recognize the error of their ways and the worth of their victim. Bonus if the MC is the victim and another bonus if a bully becomes a love interest, but neither are entirely necessary.

The only one I can think of is GTO. It's only a subplot but quite enjoyable and it fits your second bonus point.

Yeah, I read about that on Wiki. Actually, reading that was the first time I thought I'd like to see a plot centered on this. I may try to watch that series again, wasn't really able to stick with it last time I watched it (and never tried the manga so far but just might...)

Most shounen manga have these situations but they dont really dwell too long on it leading to the usual protagonist and his new friends fighting the bad guys... goku and most his friends (DBZ), yugi and his friends (yugioh), besides the usual these are a few

Shijou Saikyou no Deshi Kenichi

Yu Yu Hakusho

Eyeshield 21

Kateikyoushi Hitman Reborn!

I don't know any where the bullying lasts a long time

Note: All of these series belong to the "School Life" genre as well as the genres listed.

ACTION

Black Haze

Boku no Hero Academia

Iris Zero

ADULT

Kanojo wa Sore o Gaman Dekinai (IWASHITA Shigeyuki)

SEINEN

Lookism

SHOUJO

Note: In these series it is the main female lead who wins the bullies over (i.e. the bullies become friends) and it is usually other girls who are jealous of her being in a relationship with the most popular guy in school (i.e. the main male lead).

Kareshi Kanojo no Jijou

Note: Kimi ni Todoke may also fit here.

Suki tte Ii na Yo.

Quote from Black Skittlez

Most shounen manga have these situations but they dont really dwell too long on it leading to the usual protagonist and his new friends fighting the bad guys... goku and most his friends (DBZ), yugi and his friends (yugioh), besides the usual these are a few

Shijou Saikyou no Deshi Kenichi

Yu Yu ...

A little different I'd say. In these series on many cases the bullies get some serious comeuppance but never really change their opinions and just continue to view the character as trash. And many other cases have the bullies just suddenly show respect, maybe smile and say "you're alright". I'm really looking for something where the bullies experience some legitimate remorse for what they did and/or empathy with the victim, combined with acceptance or an attempt at acceptance toward them.

Yamada-kun and the Seven Witches fits this somewhat, though in his case it's more of a "not having friends because he's scary" situation in which one person is partially responsible for his reputation. They do wind up showing remorse, however, and the manga does analyze how it's affected them both and their relationship.

Annarasumanara's protagonist is bullied by her classmates, one of whom is a generally douchey entitled guy who grows over the course of the story and serves as kind of a love interest.

Most of Aku no Hana revolves around the protagonist being bullied/manipulated into doing the female lead's bidding, though tbh he's kind of a creep and it's not entirely unjustified. There is an eventual

time skip in which they meet and discuss their regrets towards the end, however.

I'll be looking into a couple of these at least, particularly Yamada-kun.

I feel you. I had read Koe no Katachi and I have been there. You want to experience it all over again.

I think you can try
Arifureta Shokugyou de Sekai Saikyou (Novel)
Fudatsuki no Kyoko-chan
Onani Master Kurosawa
Yahari Ore no Seishun Rabukome wa Machigatte Iru. (Novel)
+a no Tachiichi
Hikaru ga Chikyuu ni Itakoro...... (Novel)

I second Aku no Hana. Worth reading.

UnOrdinary from the Webtoon website has this, and probably will be heading in this direction.