There are fanfictions where Naruto obtains more than one bijuu because of a distortion in the chakra flow of the world. This disruption causes the bijuu to be drawn to Naruto. Also, in certain stories, Naruto's strong will and his existing bond with Kurama (the Nine - Tailed Fox) make him a magnet for other bijuu. They see his potential and willingly choose to ally with him, thus giving him their power.
Often, in fanfictions, Naruto may get more than one bijuu as a result of a deal or pact. He could make an agreement with a powerful being who has the ability to transfer bijuu powers. For instance, he might help a sage in exchange for the power of extra bijuu. Another way is through a unique genetic inheritance. Some fanfic authors write that Naruto has a hidden lineage that allows him to naturally attract and hold the power of multiple bijuu.
One popular fanfiction could be 'Naruto: The Bijuu Master'. In this, Naruto somehow manages to gain control over multiple bijuu and uses their combined powers in unique ways against powerful foes. Another could be 'Naruto's Multi - Bijuu Adventure' which focuses on Naruto's journey as he tries to balance the powers of different bijuu within him. There are also fanfictions like 'Naruto and the Extra Bijuu' where he gets additional bijuu due to some unforeseen circumstances and has to adapt to the new powers quickly.
There are fanfictions where Naruto absorbs the bijuu during a life - threatening battle. As he is on the verge of defeat, he somehow taps into an unknown power within himself and ends up absorbing the bijuu to turn the tables. This often involves a strong emotional connection and a will to protect, which acts as a catalyst for this absorption.
Naruto might be initially shocked. He has always seen the bijuu as powerful entities, not as potential mates. But being Naruto, he would likely approach the situation with his usual kindness and try to understand their feelings.
In many Naruto fanfictions, if Naruto has three bijuu, they could be the Nine - Tails (Kurama) which he already has in the original series, perhaps the Eight - Tails (Gyuki) as it has a connection with Naruto's world and Akatsuki's hunt for the bijuu. And another could be the Three - Tails (Isobu). However, it really depends on the specific plot and creative direction of the fanfiction.
The possessiveness of the fem bijuu towards Naruto can be a major plot device. It can add an element of drama and tension. Maybe the fem bijuu are possessive because they see Naruto as their only hope or as a special being. This possessiveness could lead to them overprotecting Naruto. As a result, Naruto may become more isolated from his usual comrades, which would be a significant change in the story. And this isolation could cause Naruto to develop in a different way, perhaps becoming more reliant on the fem bijuu or learning new skills to deal with their possessiveness.
His power becomes extremely overwhelming. With all nine bijuu, Naruto gains access to an insane amount of chakra. He can perform massive jutsu without much strain. For example, his Rasengan could potentially be magnified to a scale that could destroy large areas with ease. Also, his physical abilities like strength and speed would be enhanced to superhuman levels due to the boost from the bijuu's power.
Well, it makes Naruto extremely powerful. He becomes like a force that no one can easily oppose. His chakra level skyrockets, and he can use the unique abilities of each bijuu. This changes the power balance as he can now handle threats that were previously too difficult, like taking on powerful villains single - handedly.
Naruto's character develops by becoming more empathetic. He starts to understand the pain and loneliness of the Bijuu. This makes him less impulsive and more strategic in his approach to saving them. He also grows in terms of his leadership skills as he has to convince the Bijuu to follow him.
Naruto's character development in this type of fanfiction often involves becoming more mature. With the power of the Bijuu, he has greater responsibilities. He realizes that his actions now have a much bigger impact. He may also become more confident, as the Bijuu's power gives him an edge in battles. But at the same time, he has to be careful not to let the power corrupt him.
When Naruto can control the bijuu in the fanfiction, his relationships shift. His friends might be more protective of him as they know his power makes him a bigger target. The villagers who once shunned him could now either idolize him or be even more suspicious. And in the ninja world at large, other villages may try to form alliances with him more eagerly, while some might plot against him out of fear that he'll use the bijuu to dominate. Naruto's newfound control over the bijuu is like a catalyst that stirs up all kinds of reactions in his relationships.