Man, Fist of the North Star is a classic, hard to name anything that may be modern in relation to that BUT lucky for you, they are actually going to create a new adaption to the series.
Info here:
https://www.crunchyroll.com/news/latest/2023/9/12/new-fist-of-the-north-star-hokuto-no-ken-anime
Aside from that, there really isn't anything that is going to scratch that itch that's modern in my own opinion.
If you're interested in some older material, Sakigake!! Otokojuku may be a good start. Gantz MAY also be something you're interested in if you enjoy gore, post apocalyptic material, etc.. (Gantz is incredibly gorey and isn't something that I personally enjoyed because of how brutal it is, but that depends on the watcher and what they personally enjoy.
I'll just throw some out there that you may enjoy, Jojo's Bizarre Adventure, Berserk, Baki the Grapper/Baki, Claymore, Basilisk, Ninja Scroll, Vampire Hunter D, etc.
My main question that I'll ask you is this though.. What exactly did you like about Fist of the North Star? If you like over-the-top wacky martial arts nonsense, maybe Baki will be for you. If you like the idea of large, eccentric men beating the hell out of each other with weird and particular tactics and strong personality, maybe you'll like Jojo's Bizarre Adventure.
Some more modern options that I'll throw out there:
Attack on Titan, Baki (2018), Dororo, Kengan Ashura, Jujutsu Kaisen (maybe?)