You wouldn’t get anything resembling either series.
Kishimoto was much more involved with interpersonal relationships, particularly with his main character. There’s significantly more time spent on Naruto coming to terms with himself and the world in Shippuden than him fighting. Likewise you have more weight to the focused friendships for the protagonists.
Naturally you’d have much better fights, you’d probably have more streamlined antagonists & Blackbeard wouldn’t be the big bad. I’m not sure if he’d be a Sasuke analogous character but I can see Kishimoto writing him as someone straddling the line who dips his toes into either end of the moral spectrum, but ultimately helps against the world government.
We would get significantly less world building and adventure. Due to the nature of the story, we’d see more than in Naruto but we wouldn’t be getting so many islands and fantastical ideas. There would be a lot of Oda’s creativity lacking in a Kishimoto written One Piece.
I think haki would exist, but it’s either a day 1 concept or something that is exclusive to the very strongest characters, like the doujutsu were in Naruto (sans the byakugan).
If Oda wrote Naruto?
we are getting a much more drawn out story, exploring different locations—hidden villages and otherwise—guaranteed. Each village will have a distinct, if not shallow, identity we can easily associate with them. There will be more mysteries in the plot & the plot itself will be denser in the earlier arcs though gradually thin out.
we are guaranteed to get more hype moments for Naruto, Sasuke, Kakashi, maybe Rock Lee or Neji too. Oda has a checklist for his important characters and that usually lends to giving them something exciting to accomplish.
The Akatsuki would be a major threat but likely also pawns—or hired professionals—from a hidden village or other shady group. They’d be a cross between Cipher Pol & Shichibukai imo.dealt with earlier but becoming recurring later in the story.
villains wouldn’t die as quickly—good and bad—becoming returning allies. I can see Orochimaru & Kimimaro becoming a stand in for Crocodile and Daz Bones for example.
Lore like Kaguya would be hinted at around the Pain arc, Kaguya would be shown or more strongly foreshadowed by the start of the war arc, making her eventual emergence way way less grating.
We’d likely get more Hashirama lore, if he’s our Garp/Whitebeard/Xebec stand in.
The negatives? Fights are not gonna be good. They can be emotional and cinematic but no one would be talking about Rock Lee & Gaara for the choreography, we wouldn’t see intelligent fights like Sasuke vs Deidara. They’d be fine and good but it would be more about style than the actual physical conflict.
Oda does do deaths but they are almost always flashback deaths. So I think we’d get Jiraiya’s death, a few minor side characters (Darui, Ao, Kankuro) instead of anyone Major.
chakra is not explained in depth but there would be many more variations of it and it would be an evolving system. I can see senjutsu, yin-yang release, even doujutsu getting more build up and shared more as a Conqueors haki substitute or equivalent in his writing.
It would be interesting to read both stories written by the other for the novelty but they’d be lacking what made each story shine.
Both authors could borrow heavily from the other to improve their own stories though, in my opinion;
Naruto desperately needed more world building pre-4th great Shinobi world war, which Oda would give it in spades.
One Piece is in dire need of emotional weight and significant relationship overhauls between the main cast.