I never really thought of that, but it would make a pretty good fanfiction story.
My only problem is if this alternate scenario is the larger part of the story, how the story be told in an entertaining way without becoming depressing when Simba believes when he can't live up to his father's legacy. Beacause we, as the audience, want to root for him. Not to mention, how could the producers throw in musical sequences since music plays a major factor in TLK series?
So, I mostly agree with morning-star that I (kind of) like the movie just the way it is, but Simba's story and Kiara and Kovu's romantic story could have connected together, and supported each other so we get an equal balance of the original and new characters.