In fanfiction, a human can be integrated into the world of The Lion King in multiple ways. One way is if the human is a reincarnation of an important lion from the past. They would have the memories of their past life as a lion and use their human knowledge to make changes in the present Pride Lands. For instance, if they were a reincarnation of Mufasa, they could use their human understanding of leadership to make the pride even stronger. Another possibility is that the human is an outcast from their own human society and finds a new home and purpose among the lions. They could bring new ideas like building shelters or new ways of storing food. Also, a human could be a magical being who is drawn to the Pride Lands to help restore balance. They could use their magic to bring back the lushness of the land that was lost during Scar's rule.
A human can fit in by being a lost traveler. For example, a human could be on an expedition in Africa and accidentally stumble into the Pride Lands. They would then have to learn the rules of this new world quickly. Another way is if the human has a special gift, like being able to communicate with animals. This would allow them to interact with the Lion King characters easily. The human could also be a spirit guide of sorts, sent to help the lions during a difficult time.