The lab could hold secrets about Orochimaru's past. Maybe there are records of his early days as a ninja, his training, and what led him to become so obsessed with immortality. Naruto might stumble upon these records and gain a better understanding of his enemy.
One interesting take in a fanfiction could be that Naruto finds Orochimaru's lab while looking for Sasuke. He enters the lab full of determination to bring Sasuke back. Inside, he sees the remnants of Orochimaru's attempts to transfer his soul or create powerful hybrids. This discovery makes Naruto realize the full extent of Orochimaru's madness and the danger Sasuke is in. It also gives him more motivation to not only rescue Sasuke but also put an end to Orochimaru's evil plans once and for all.
Another scenario could be that Naruto discovers a hidden chamber in the lab. Inside, there are test subjects that Orochimaru was using for his immortality experiments. Naruto feels sympathy for them and tries to rescue them. However, he has to figure out how to deal with the various traps and security measures in the lab first. He might call for help from his friends like Sakura or Kakashi to assist him in this difficult task.
Naruto could find a hidden passage in Orochimaru's lab that leads to a secret chamber. In this chamber, there are ancient scrolls with powerful jutsu that Orochimaru was planning to use for his own gain. Naruto might decide to take these scrolls away to prevent Orochimaru from getting his hands on them again.
One secret could be the formula for a super - strong chakra - enhancing potion. This would give him an edge in battles against tough opponents.
He might discover some long - lost jutsu. Ancient labs could hold secrets to powerful ninja techniques that have been forgotten over time. Maybe he'll find scrolls with unique chakra - based abilities or old - fashioned weapons with special powers.
He might discover some new and powerful jutsu hidden there. Maybe it's a lab created by an ancient ninja clan that was lost to history.
Well, Naruto might first try to free Kaguya. He is a hero after all and doesn't like to see anyone chained unjustly. But he would also be very cautious because Kaguya is extremely powerful and was once a great threat. He could try to communicate with her to understand why she is chained there in the first place.
A character like Sasuke could be further developed in Orochimaru's lab fanfiction. He might struggle more with the dark knowledge he gains there and eventually find a way to use it for the greater good. Another character could be a new, made - up test subject who starts out as a scared victim but then becomes a strong fighter after enduring the experiments. And Kabuto, who is already associated with the lab, could have a change of heart and try to atone for his actions in the lab.
He might uncover a hidden weapon that could change the balance of power in the ninja world.
It could hold the secret to summoning an army of super - strong creatures.