You know, I would have nice if Simba had both a boy and a girl. There were plans for it with the names being Chaka and Shani, though Chaka was written out, and Shani was renamed Kiara. It's true the son character would have useless in the story, but if I were to have wrote the the story, the son was to be Simba's heir, and Kovu would try to kill him before he can kill Simba. Meanwhile, Kiara tries to find out what her place is in the Circle of Life. The ambush happens, and both prides lose their eldest son; for Zira, it's Nuka and for Simba, it's his son. Kovu gets exiled, and the rest of the movie's plot follows after.
Another version of my fanfiction is that only Nuka dies, but Simba's son returns home weakened trying to defend his father who was the target of the ambush. Kovu gets exiled, and the rest of the movie's plot follows after. Eventually, Simba's son and Vitani become a couple.
As for the topic, should? Probably not. The father-son relationship would have been redundant, but the writers could have continued the father-son relationship that already existed between Mufasa and Simba with Simba trying to live up to his father's legacy. Also,
The Lion King movies needed more female presence, and I believe having the story evolve around Kiara was created to accomplish that goal. Finally, the story was inspired by
Romeo & Juliet which mostly caters to a female audience so a female protagonist was needed to support the story, though a
topic I created a few years ago questions the possibility of it revolving around a male protagonist.