I hate the theories of Kopa dying too. In my fanfic he appears as a rogue who was nearly killed by Zira but survived and comes back to the Pride Lands, and falls in love with Vitani.
Some fan-theories about Kopa:
-he was killed, or nearly killed by Zira.
-Zira was banished for killing Kopa.
-Kopa and Vitani were a love couple.
-Kopa returns to Pride Rock as a scarred adult and fights Kovu to claim the throne.
-Kopa was friends with Nuka as a cub.