I'm a simple spirit or I'm flesh and blood...
But I'm alive,
I am so alive,
And I feed on the fear
That's behind your eyes.
And I need you
To need me
It's no surprise
I'm Alive, from Next to Normal
I suppose you can take this as my commentary on the whole "Kopa vs. Kion" debacle. Kopa is like the fandom ghost - he doesn't exist and he's not alive, or is he? Either way, he will haunt TLK fans forever! *cackle*
So, the idea behind this tale is inspired by Next to Normal; Kopa did indeed die as a young cub, before Kion was born. It was a dreadful blow to the royal family and it affected Simba deeply. So much that something snapped and suddenly he thought he saw and heard Kopa everywhere. He would sit and talk with the cub, certain that Kopa was actually there and confused as to why no one else could see him. Not only did Kopa stick around, but he also aged up and always appeared as the age he would have been if he had lived. Eventually, as time passed, the worried pride were able to convince Simba that Kopa was gone and that this "ghost" wasn't real. The king learned to ignore the ghost's calls for attention and was able to return to his old self... sort of. Because Kopa wouldn't leave him completely alone, and would sometimes come around to remind his father that he was - indeed - so alive... Is Kopa truly a ghost or just a figment of Simba's imagination? They would never know.
And, yes, I suppose it'd make more sense historically and according to lion prides that Kion, as a male, would become king regardless of older female siblings. But it wouldn't look very good in a Disney show of today to suggest that Kiara isn't the heir despite being the oldest, simply because she's female. So I'm glad that Disney has at least taken a modern route there
You know what? That would work so much better. The Lion King 2 timeline already didn't mesh well with the original, adding Kion into the mix only made the continuity of it much worse. They wouldn't have been able to use Zira and the other Outsiders in the series if it were a sequel to the second movie, but they still could have introduced other prides into the surrounding area where the Outsiders no longer resided.