A third concept might be the human's relationship with the main characters. They could form a special bond with Kion, perhaps teaching him about human values like friendship and loyalty in a different way. Or they could have a rivalry with one of the villains, adding a new layer to the story.
One interesting concept could be a human having unique abilities that interact with the powers of the Lion Guard. For example, the human might have a special connection to the magic of the Pride Lands, like being able to communicate with the animals in a different way than the normal animal communication in the Lion Guard universe.
Another concept could be the human's role in the conflicts. Maybe the human is from a different land and gets caught up in the battles between Scar's followers and the Lion Guard. They could bring new strategies or weapons from their own culture that change the dynamics of the fight.