There's never any doubt that the Naruto series is going to be centered on the titular shinobi. Though there are plenty of filler episodes of the anime that allow other characters to get the spotlight, it's always Naruto's story. That being said, there are a lot of ing characters that bring their own version of main character energy to the narrative.
Sometimes, that energy is so strong that the characters get a spinoff. Rock Lee, for example, did become the main character of his own story in a spin-off anime. Other characters, however, aren't exactly as lucky.
Konohamaru
The Naruto franchise loves its character parallels. There are so many characters whose story mirrors Naruto's own, but a slight twist gives them a very different upbringing than him. One of those is Konohamaru Sarutobi.
Konohamaru is the grandson of the Hokage at the start of the series. His parents are of the Anbu, meaning they spend most of their time on secret missions instead of with their son. He acts out as a child, wanting as much attention as possible from his grandfather, even trying to regularly best him in matches right up until his death. Konohamaru would have made for a fascinating protagonist, as he grew up learning what it meant to be at the center of the village's attention in a very different way than Naruto.
Shikamaru
Shikamaru is a fan-favorite character, largely because of the immense amount of growth he experiences in the series. While he would absolutely protest his main character energy as a child (citing his self-professed laziness, no doubt), he definitely has the potential to be a central figure in his own tale.
Shikamaru's arc sees him turn into a real hero. He's chosen for his logical mind to be a team leader, despite him not wanting the responsibility. He also falls for the very girl he calls "troublesome" when he has to fight her in a match. Getting to see more of his point of view during his growth would have been great for the audience.
Iruka
The audience sees the Naruto story told from the point of view of the students, but the story from the point of view of the teacher could be great. Iruka would be the focal point if that was the case.
Not only does he teach the entire group of genin at the Academy, but he's also a long-suffering er of Naruto. He's there time and again when Naruto finds himself in trouble as a kid, even putting his own life on the line for Naruto despite being frustrated by him so often.
Tsunade
Tsunade's journey through the series is an incredible one that fans only get to see glimpses of. She grows up expecting to be a shinobi that can change the world, but trauma makes her run from the village she calls home and not look back after fighting in a war.
Tsunade doesn't just redeem herself in the eyes of the village citizens who had dismissed her, but also in her own eyes. She overcomes her phobia of blood to fight, she becomes engaged with dreaming about the future again thanks to Naruto, and she takes on a new student in Sakura. Tsunade's journey would make for its own compelling drama.
Kabuto
If any character has main character energy amongst the villains of the series, it's certainly Kabuto. Orochimaru might be afraid of inevitable death, but Kabuto wants his mentor's power - and to practically steal his life right out from under him.
Kabuto is absolutely the snake in the grass of the story, acting like an ally to different characters until revealing himself as a villain at the last minute. He would make for a fascinating main character, especially as his journey ends with him attempting to make amends and running a home for orphaned children in the Boruto series.
Ino
Ino is the most fashionable of Naruto's generation, ready to give anyone advice on their hair or clothes at a moment's notice. Despite appearing incredibly shallow, however, Ino has a good heart and is a fierce fighter. There are a lot of layers to her that the show really doesn't have the time to explore since she's not the focus.
A deeper look into Ino that's centered on her trying to balance working at the family flower shop with her own shinobi studies would be a welcome one.
Neji
Neji has one of the biggest turns in the series. He goes from hating his Hyuga relatives and trying to kill Hinata during their Chunin Exam match to protecting her in battle - and giving his life for her.
Watching Neji's anger build and then thaw in a story of redemption and tragedy would be a perfect shonen tale. Of course, Neji's story would come to an end long before the major story in Naruto, but it still feels like a concept that's worth exploring in the broader Naruto world.
Tenten
Of Naruto's generation, Tenten is really Naruto the character that is left out the most. She doesn't even get a last name! She does have a unique command over a variety of weapons though, which makes her really fun to watch in battle. It's clear that she's put in the work to be a strong shinobi.
Tenten's place in the filler arcs of the anime, as the team member most frustrated with Neji and Lee's competitive nature and devotion to training, makes her sympathetic to the audience. Seeing her as someone who gets frustrated with her teammates easily opens up a lot of comedic possibilities for her story. It's easy to picture her as the lead wondering why no one else can be as even-keeled as she is.
Gaara
Like Konohamaru, Gaara's story is one that mirrors Naruto's in a lot of ways. Gaara is also a jinchuriki, but as a member of the ruling family in Sunagakure, he's raised to be a weapon. Gaara spends his life being feared, but also in a position of power, vastly different than Naruto being the village outcast.
It would make for a fascinating arc to see Gaara grow up, only really becoming open to friendship and the love of his siblings once he interacts with Naruto and other Konohagakure shinobi. As it is, the audience only gets to see bits and pieces of Gaara's story.
Itachi
Itachi's life is so vastly different than any other Naruto character that fans have to wonder why his story hasn't been told on its own since the big twist to his character was revealed. He massacred his entire clan to save his village, only sparing his little brother, and began a life on the run as a villain wanted all over the shinobi world.
That renegade story of a misunderstood hero, living amongst villains to protect others would make for a hit anime series. His story might end in tragedy, but he accomplishes his goal of helping to keep Sasuke and his village safe in the end.