A seinen to be good doesn't need to be a gore feast, just erase the disgusting levels of plot power and sugar coats everything gets.
Just to try an original seinen piece I would make Boa Hancock the main character. She has her background from One piece just that it is actually not sugar coated. She gets her usual personality minus the gags, love for Luffy (he actually doesn't exist in this story) as a plus she is internally scarred from her past (she can usually cover it but not always), in a way she gets a bit like Guts from Berserk. She is pretty smart like in the manga but also way more devious and ruthless. Of course has no problems to kill when needed. Her aim is revenge (at least at first, then she could grow, a bit like Musashi from Vagabond this time) at the CD who destroyed her childhood so her enemy is the WG. Maybe Im and the gorosei below him can be kept as the "real villain" reveal later on.
The general plot could be... She travels the seas in order to get infos about the CD (CDs) she wants to kill and at the same time get stronger cause she knows she will have to fight the WG for the CD head (so get strong enough to fight admirals in other words). She doesn't hate the pirates by default but despises them generally (cause they are generally scum) but can come to terms with them if they are not total scumbags or if she really needs their help (I would also put some moral choices for the MC at least once every arc)(even sexual ones like for Griffith and the noble, but not make that an excuse for fanservice). Eventually she comes to know there is a CD in Dressrosa... At this point some Togashi level arc where all kind of people and interests collide, making weird alliances (York shin arc, Hisoka and Gon and Killua)(Boa despises Doffy cause he is Doffy but at the same time shares the hatred for the CD, even if Doffy hates everything in the end)(anyway a seinen Doffy and this seinen Boa interaction would be really interesting for me). Eventually why not put in a psycho mute Killer in the story as well (he was in a ruthless and mad crew, Kidd's, but they in their madness decided to challenge an emperor and Killer was the only survivor, he suffers PTSD and can not talk anymore, just scream at night for nightmares maybe). Then maybe Boa original crew are her and her sisters plus the amazons, I would make a sister eventually die since they are redundant, also it adds drama, same for a good number of the amazons (eventually meet some sea horror unrelated to the story/OP world and have a berserk style eclipse). Also why not add a lot of mysteries that remain unsolved and unexplained (a bit like the drifting dragons manga, I like the aura of mystery that manga has). Then I don't know, I think it is enough. For end of series it would also be interesting if she is close to her objective, she had located the CD she seeks and also is strong enough to reach and kill him but just before she is about to start BB actually overthrows the WG and starts his mad reign who make the CD rulership pale in comparison. She at this point has to kill hik since he is even worse than the CDs and is making a world based on his ideals. Oh, also keep inherited will and all that is around it would be fun (that for me was a strong point of one piece) but at the same time there is no real plot power at work her and sometimes you have to take difficult choices, sometimes there even isn't a right choice but only wrong ones, so inherited will is difficult to bring on here.