A new character could be introduced as the antagonist. For example, a powerful and greedy hyena who wants to take over the Pride Lands. When Mufasa and Simba meet, they have to team up to fight this new threat. The hyena might have some special powers or a large army of his own followers.
In this fanfiction, it could be that Mufasa is brought back to life by some magical force. When he meets Simba, there is an initial shock and then pure joy. They then set out to restore the Pride Lands to an even greater glory. Mufasa shares his long - lost knowledge about the hidden parts of the Pride Lands, and together they form a stronger bond as father and son while working towards the betterment of their kingdom.
In some lemon fanfictions, their relationship might develop more quickly. For example, Simba could have a series of adventures where he discovers more about his father's past in a very short time. This new knowledge could make him idolize Mufasa even more and strive to be like him sooner.
It could be a cruel dog catcher. He's always on the lookout for stray dogs like Scamp. He sees Marinette trying to protect Scamp and tries to outwit her to catch Scamp. He doesn't care about the bond between Marinette and Scamp and is just focused on doing his job, which in this case, is seen as the 'bad' thing.
It could be a creature born out of chaos. This being might be drawn to the unique energy created by the meeting of Atreus and Calliope. It tries to consume their combined power, causing destruction in their path as they struggle to stop it from achieving its goal.
A Sith Lord who has been manipulating events from the shadows could be the antagonist. This Sith Lord might be trying to use Rey's connection to Anakin to bring about the return of the Sith in a new and more powerful way. He could be using dark side illusions to make Rey and Anakin distrust each other.
Maybe Orochimaru could be the antagonist. He hears about Naruto's encounter with Yagura and wants to use their connection for his own evil experiments. He might try to capture Yagura again (if he was once under Orochimaru's control) or manipulate Naruto into doing something that would harm Yagura and the relationship between the two villages.
One possible plot twist could be that Simba, instead of facing Scar alone in the final battle, teams up with an unexpected ally from the outskirts of the Pride Lands. Maybe a group of outcast animals who have their own reasons to oppose Scar's tyranny. This would change the whole dynamic of the classic story and add a new layer of complexity.
Well, perhaps Scar would initially be defensive when meeting Mufasa in the afterlife. But Mufasa, being the noble lion he was, might offer Scar a chance to redeem himself. This could lead to a transformation in Scar, where he starts to understand the error of his ways. It could also show how the power of forgiveness can exist even in the afterlife.
The main characters are obviously Simba and Nala. Simba, the future king, is brave and has a strong sense of justice. Nala, on the other hand, is smart and independent.
It could be a powerful warlock who has found a way to manipulate time and space. He wants to stop Paige and Prue from joining forces because he fears their combined power. So he creates various obstacles and illusions to keep them apart.