Well, perhaps he can't slow down time for living things only non - living things. This would mean that his opponents can still move and attack him while he's trying to use the power on their weapons or the environment. Also, the more he uses it, the more it messes with his own perception of time, and he might start to lose track of real - time events.
One creative use could be in training. He can slow time to practice his jutsu more effectively. He'd have more time to perfect his movements and chakra control, making him a much stronger ninja in a shorter period of actual time. Another use could be to save people from disasters. If there's a building collapsing, he slows time, runs in, and gets everyone out safely.
Another limitation might be the amount of chakra it consumes. Using this power too much could drain him quickly, leaving him vulnerable in the middle of a battle. Also, if there are opponents with abilities to disrupt chakra flow, they could potentially stop him from using his gravity control.
In some fanfictions, Naruto's hidden power could be the ability to control a powerful and ancient force. For example, he might be able to harness the power of some long - forgotten bijuu or a mysterious energy that was sealed away in the past. This would give him abilities that are on a completely different level compared to his normal skills.
One scenario could be that Naruto uses Shirou's tracing ability during a fight with Sasuke. He might be able to quickly understand and counter Sasuke's new jutsu that he developed during his time away. Another possibility is that Naruto uses projection to create a huge Rasengan - like structure that can cause massive destruction, similar to how Shirou projects his weapons on a large scale in some situations.
It could be an ancient inheritance. Maybe Naruto is a descendant of a long - forgotten clan that had special soul powers, and these powers have been dormant in his bloodline until now.
In a Naruto - Goku fusion fanfic, the power - ups could be really exciting. Since Goku has his Zenkai boosts, and Naruto can gain more chakra and power as he progresses in his ninja journey, the fusion might have a new type of growth system. For instance, every time the fusion is in a tough battle, it could gain a permanent boost in strength, speed, and energy. Also, the fusion could potentially access a new form that combines the Super Saiyan transformation and the Nine - Tails' chakra mode for an insanely powerful state.
It could also be a result of a powerful curse or blessing. For instance, he might be cursed by an old enemy with the ability to control time as a side - effect, but he then learns to master it. Or, it could be a blessing from a sage or some other powerful being who sees Naruto's potential and gives him this unique ability to help him save the world.
In this fanfiction, it might be due to a combination of his strong will and a unique set of circumstances. Naruto's willpower has always been a defining trait. If in the fanfiction he faces an extreme threat to the entire universe, his will could be the catalyst that unlocks this planet - destroying power. Also, it could be that he has gathered the power of all the tailed beasts in a new and more powerful way than in the original story, which gives him the ability to destroy a planet.
Another plot twist could be that the powers of the Dino Thunder Rangers and the ninja abilities in Naruto are somehow related in a way that no one expected. Maybe the origin of ninjutsu was actually influenced by the same energy source that gives the Rangers their powers. This discovery could change the way both groups view their own powers and could lead to new alliances or conflicts within the fanfiction.
A plot twist could be that the main villain in the fanfiction is actually a character from one universe who has been corrupted by the power of the other. For example, a Naruto character who becomes evil after getting a hold of some Ninja Steel power. Or vice versa, a Power Ranger who is influenced by the dark chakra in the Naruto world.