I always loved Kopa, actually I only became a true Lion king fan, when I started to see fanarts of the lion king characters, and I got curious to know who was this Simba's lost son, and search more about the lion king story. So here are one of my theories of Kopa and how I think he looks like :)
1. Kopa & Vitani
I support the shipping between these two, I think they would make a lovely couple, maybe better than Kovu and Kiara. I just don't like when people start to think that Zira "killed" Kopa because of his relationship with Vitani >:
2. Killed by Zira?
One of the most "famous" theories about the disappearance of Kopa. Of course it makes a great sense if you think about it. Zira might have been exiled for killing Kopa. But even so, I never really liked this theory, although it would make a great movie.
3. Parallel Universe
Adding some of my conclusions, I think that Kopa really exists, but in a alternate universe, of how Simba's life could have been. Kopa never existed in The lion king 2 universe, only in the books. He has a story and a life, but he don't have a sister, Zira never existed, Vitani never existed, Kopa never died in his universe, he just grew up and lived his life, he never meet Vitani because she's from The lion king 2 movie universe, beside the fact that the Six new adventures books where Kopa appears, has been launched before the movie of The lion king 2, what makes the world of The lion king 2 movie, be an alternative idea, and the world of Kopa be the real story. Or maybe everything is just a dream of a lion. Maybe you are a dream of another person in another world. Maybe your just a baby dreaming about the future life, or you are dead dreaming about your past life. SWEET DREAMS YOU ALL :)
Original image and characters (C) Disney