Just something that kind struck me as odd is that if the Roar of the Elders is such a powerful tool, why don't kings have it? Not only is it a very nice weapon to have should things go bad, but it also gets rid of the requirement of having a second-in-line to the throne.
Just look at the main Lion King movies: When Simba is a cub, since he is the only son of Mufasa and Sarabi, he doesn't have a brother to be in charge of the guard. It has been already established that Scar lost his ability to use the roar in a failed coup, so it would only make sense for Mufasa to move that position to the next generation... except that his only son was Simba, making it impossible.
Then when Simba finally becomes king, it is still a long time where he is in control, but with no one to use the Roar of the Elders. Imagine if he was able to use it against the hyenas during the battle for Pride Rock? Or what about using it against the Outsiders? It is only after Kion is born that there suddenly is someone capable of using the roar, so in many ways, that position was left vacant - therefor the Pride Lands very vulnerable - for just about an entire generation.
Let's ignore the obvious real-world reason: that the roar is made up for The Lion Guard specifically. So of course the movies, which came before, won't have it, and The Lion Guard needs to come up with a reason why it won't be Simba who would be able to use the roar, so Disney came up with the second in line thing.
Ignoring that, what could be a possible reason why the king would not be allowed to use the roar, despite it being a very good thing to have, especially for the king?
I am going to go with how they described it in the 16 minute presentation: that the Roar is so powerful that it is also a very tempting thing to use, even when inappropriate. So especially for a king, they will all too often be pressured by their duties to use it when they should not, so it was decided that rather than the king be the one with the ability, someone close to them should - the second in line in this case.