He may gain better control over his chakra. The toads are known for their chakra - related knowledge, so they could teach Naruto advanced chakra control techniques.
Naruto might learn some new summoning jutsu. Since he is training with the toads, they could teach him more powerful and unique summoning spells. He could also gain enhanced physical strength and agility. The training with the toads may include rigorous physical exercises that improve his overall combat capabilities. Additionally, he may develop a better understanding of nature energy, which is a very powerful and mysterious form of energy in the Naruto universe.
One of the new abilities Naruto could gain is better information - gathering skills. Since ANBU are often involved in intelligence work, Naruto could use this in the Chunin Exams to find out his opponents' weaknesses. Also, he could learn how to use chakra in more creative ways, like creating more complex and powerful ninjutsu. This would set him apart from other candidates in the exams as he would have abilities that are not commonly seen among genin.
In fanfiction, Naruto could gain enhanced sensory abilities. Since toads are known for their connection to nature, Naruto might be able to sense things from a greater distance, like the movements of enemies or the flow of chakra in the environment. This would give him an edge in battles.
He might gain better chakra control. Kakashi could teach Naruto how to precisely manage his chakra, allowing him to perform jutsus more efficiently and for longer periods. For example, Naruto could use his Shadow Clone Jutsu without exhausting his chakra so quickly.
He probably gains the ability to control the chakra of the Kyuubi more freely. This could mean enhanced strength, speed, and reflexes. He might also be able to use some of the Kyuubi's unique jutsu like the Tailed Beast Ball. His healing factor could be improved as well, allowing him to recover from injuries faster.
He might gain the ability to cut through almost anything. The black lightsaber, being a powerful weapon, could enhance Naruto's already formidable combat skills. He could use it to slice through strong jutsu defenses with ease, something that normal kunai or shuriken couldn't do.
One significant ability he could obtain is the power to enter a special sage mode unique to fox sages. In this mode, his appearance might change to incorporate more fox - like features. His speed would be incredible, enabling him to dodge attacks with ease. His ninjutsu could also be enhanced, for instance, his Rasengan might become larger and more powerful, infused with the energy of the fox sage.
He might face challenges like mastering new jutsu. The toads have unique techniques that Naruto has to learn, which could be physically and mentally exhausting.
In such fanfiction, Naruto may gain the ability to heal himself more quickly. Since the tailed beasts possess a great amount of life - force energy, Naruto could tap into that to accelerate his own healing process. His mental abilities could also be enhanced. He would be able to withstand greater mental pressure and use the tailed beasts' collective wisdom to make better decisions in battles. Moreover, he could potentially create a chakra shield that is extremely strong, using the combined chakra of all the tailed beasts to protect himself and his allies.
Naruto would gain the ability to copy jutsu. With the Sharingan, he could watch an opponent's movements closely and replicate their techniques. This would be a huge advantage for him in battles.
He might gain the ability to manipulate ice like Haku. This could be a very useful addition to his existing skill set, especially in battles against fire - type opponents. He could create ice clones instead of the regular shadow clones, which would be more durable in certain situations.