One challenge could be communication. Since lions communicate in different ways like roaring, growling and body language, it would be hard for a human to fully understand and be understood at first. For example, a simple warning growl might be misinterpreted by the human.
Surviving in the wild is another big challenge. Humans are not adapted to hunt like lions or find water sources in the savannah. The human in the fanfiction would have to learn quickly how to avoid predators, find food, and build shelter. In a lot of these stories, the human has to rely on the kindness of the lions to teach them these survival skills.
Acceptance within the pride can also be difficult. Lions have a hierarchical social structure. A human, being so different, might face resistance from some members of the pride who don't understand why this strange creature should be part of their group. They may see the human as a threat to their way of life or a burden on their resources.
One major challenge is the physical pain. As the body changes from a human form to a dragon one, bones have to break and reshape, muscles have to grow and expand rapidly. This can be excruciatingly painful. For example, the elongation of the spine to support a long dragon tail would be like a searing pain throughout the back.
One challenge is communication. Dragons might not communicate in the same way as humans. So the human - turned - dragon has to figure out how to interact with both humans and other dragons. Another is dealing with the new instincts. Dragons have different survival instincts, like hoarding treasure or being territorial, which can conflict with the human's former way of life. They may also face threats from dragon hunters or those who see them as a threat to the existing order.
One challenge would be dealing with Spike's reputation. If other characters know Spike well, they might expect the reborn human in Spike's body to act in a certain way. The reborn human would have to constantly prove that they are different. Another challenge could be the physical cravings of a vampire. Since they have a human soul, they might struggle more with the urge to drink blood.
Communication would be a major challenge. Since Nala is a lion and the human doesn't speak the same language as lions, they would have to find ways to understand each other. This could involve a lot of body language and trial - and - error.
Momonga would also face challenges in terms of adapting to the human lifestyle. He has to deal with things like finding shelter, getting food, and maintaining his health. In the game world, these aspects were not really relevant in the same way. He might not even know how to cook or find a proper place to live. And he would have to learn about money and how the human economy works, which is a whole new and complex system for him.
A human in the Lion King world could cause quite a stir. First of all, the natural order of the Pride Lands would be disrupted. The animals have their own way of life based on instincts and the Circle of Life. The human might introduce new concepts like building permanent shelters or using tools. This could change how the animals view their world. They might start to question their traditional ways. For example, Simba might be curious about the human's ability to communicate complex ideas and might want to learn from them. However, some animals like Scar might see the human as a threat to his power and try to get rid of them.
Cultural challenges would be a big deal. The Yautja have a strict honor code and a very different social structure. The human would have to learn their ways of hunting, which might involve killing other creatures in a way that is not acceptable in human morality. Also, the Yautja might not have the same concepts of family and friendship as humans do, so the human would have to adapt to a rather cold and practical social environment.
Survival would be the main challenge. Lack of breathable air is a big issue. The human would need to figure out if the planet's atmosphere is suitable or find a way to create or purify air. For example, if there are certain plants on the planet, they might be able to use them to produce oxygen somehow.
One big challenge for Benhypno in human form would be dealing with his memories and instincts from his alien self. He might have flashbacks or urges to act in an alien way when he's in a tense situation. For example, if he's in a fight, his first instinct might be to use his full alien powers, which he can't do without revealing himself. Also, he may face the challenge of making friends. Since he has a different background, he may find it hard to relate to normal human teenagers and their interests.
They could face hostile monsters, difficult puzzles, and the struggle to find a way back to the surface.