Jujutsu Kaisen ended its manga run by having a lot of flaws that disappointed its readership, but one correct decision by author Gege Akutami was not dropping his protagonist, Yuji Itadori, into a relationship with no build-up, with the final scene with Yuko Ozawa not changing that in the long run. This is an example that One Piece author Eiichiro Oda could follow for the ending of his story, and not give Monkey D. Luffy a relationship.
The character of Luffy has been shipped for a long time in the One Piece community with Boa Hancock, who is canonically in love with him, and also with the navigator of his crew, Nami, although the latter has yet to show a romantic interest in him. However, Oda could follow the example of Jujutsu Kaisen and simply not give his protagonist a relationship, which, on paper, would be the most logical outcome for him.
Disclaimer: This article contains spoilers from the manga/anime and reflects the opinions of the writer.
Explaining why the Jujutsu Kaisen ending could help Oda with the conclusion of Luffy's character in One Piece

Even though Gege Akutami's manga ended with a lot of underdeveloped characters and plot points, the vast majority of the fandom was happy with Yuji Itadori not getting into a romantic relationship with Yuko Ozawa or anyone else for that matter.
That is because the protagonist didn't have a developed romantic connection with any member of the cast, so the outcome would have felt forced and disappointing.
That is something that Eiichiro Oda could do in One Piece with Luffy's character by having him end the story without a romantic partner, even if Boa Hancock has shown a prominent interest in being with him. Furthermore, he has also been shipped heavily with Nami, even though the latter has never given him any signs of a romantic relationship.
Either way, while Yuji was at least seen showing an interest in women, as evidenced by his conversations with Aoi Todo and Ozawa's flashback, Luffy has never had an attraction toward women or men in the entirety of One Piece. That could change before the story's conclusion, though, but with the characterization that has been given of the protagonist thus far, Oda is better off not going in that direction.
The issue with forced romantic relationships

The example of Jujutsu Kaisen shows that manga authors don't always have to end a popular battle shonen with the main character in a relationship, which is something that would fit with One Piece.
Especially considering that Luffy has shown no interest in Boa Hancock and Nami in that manner, which is something that could feel forced on the audience, and would be the question of why Oda didn't develop them in that front before.
Speaking of the latter, another popular series, Naruto, was heavily criticized for the titular protagonist and Hinata Hyuga ending up together and the same criticism went for Sakura Haruno and Sasuke Uchiha. The issue is that author Masashi Kishimoto didn't take the time to develop their romances, thus feeling unearned and unnatural with the story's progression.
Final thoughts
All in all, Jujutsu Kaisen and Yuji Itadori's conclusion is something that Oda should take as an inspiration for the end of Luffy's journey in One Piece. Romance is all well and good, but there are characters not suited for that, and the captain of the Straw Hat is a very good example of that.
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