
If they did make that, I'm afraid to say it would have endless flaws. For one, Disney is rarely creating traditionally animated films and probably wouldn't bother doing another. Based on what most Kopa fanatics believe to be true, he ends up with a terrible fate as a cub being killed by Zira. Now, that doesn't sound very Disney-friendly, does it? Even if they did make Kopa defeat some villian (Obviously not Zira since she appears in the sequel; another flaw.), this would still leave the wonder on why they didn't add him in the sequel.
If Disney chose him to run away and make his own pride, that would seem kind of pointless and leaves an almost cliffhanger in it's abscence. Simba would be worried about Kopa and have no idea what happened to him, and the movie just wouldn't turn out well. Also, if Disney created another film after the sequel where Kopa comes back, they would have to tell how and why he left. (If he actually survived. Otherwise we'd have a zombie lion, lol) And if so, who would the bad guy be since all the prides are united? Sure a rogue could come in, but that would hardly make a fight, just one lion. They could make the villian get a group of animals and rebel against the Pridelanders, but for one, why would he? And for two, isn't that a lot like Scar and the original story-line of the first film?
I hate to be what you call a "dream-killer", but I'm afraid it's the truth, and I'm just trying to look at it from all angles.

And, for the 6737697627079267000000th time, Kopa is not in the movies.