origin of Scar's scar has been explained before, but The Lion King 2 will likely have a new take on the story, and it should further explore Mufasa's treatment of his brother.
In the Disney storybook A Tale of Two Brothers, it's explained that Scar's real name is Taka. Yet by the events of The Lion King, he's already widely known as Scar, with no trace of his birth name remaining. It's heavily implied, but never made explicitly clear, that the scar is a result of Mufasa injuring him in a challenge, but the movie never addresses his new nickname. Mufasa's treatment of his brother likely contributed to his radicalization, and that injury should be a major part of The Lion King 2.
Ridiculing Scar for his deformity by calling him Scar instead of Taka - especially when Mufasa is the one who gave him that injury - is out of character for the otherwise thoughtful king. Even if it happened when Scar challenged Mufasa, the reductive nickname is probably part of why Scar hates his brother so much, and a major reason for Scar's later treachery. If the implication in The Lion King is that Scar's injury was caused by his brother, than Mufasa's subsequent cruelty with the nickname is a sign that he's a much more complicated character than the all-knowing, wise king.
Mufasa's treatment of his brother is one of his biggest failures, both as a hero and a king. Mufasa's cruelty to Scar directly contributes to his own demise at the hands of his brother, and is the character's biggest flaw by far. The cruel nickname comes off as a toxic, alpha move from Mufasa, and the parts of his characterization that led to that decision have never been properly examined. Angling The Lion King 2 as a Mufasa prequel is an opportunity to add complexity to the king and examine his failings.
Contrary to his portrayal in The Lion King, Mufasa is clearly capable of making difficult - and wrong - decisions, and Disney should explore that in the prequel. Finally fully explaining Mufasa's backstory, the origin of Scar's nickname, and the way he got his famous injury would allow Disney to continue diving into the complicated relationship between the brothers. Mufasa's biggest failing is his relationship with his brother, and the way that he treated him like an outcast - thus turning him into one - should finally be addressed in The Lion King 2.
