I like your idea, but disease kind of sounds like old age to me. If you ask me there's two ways on how Simba could die.

1. He would die of old age along with the rest of the cast. That way, the movie could be about the future genereations of the Pridelanders. I think the characters are too worn out to make another appearance, and good thing too! I hated the sequel!

2. Simba should die protecting Kiara or one of her cubs through a Xanatos Gambit plot set by Kovu's real father. I think that knowing that he would die and then accepting it is a noble thing to do, and very acceptable for his character, since Mufasa died almost the same.

I like your idea.
Have you heard of the Kimba controversy? I actually liked Kimba first and then got into the Lion King. I think it's the best ripoff ever made. Anyway, if Disney borrows Kimba's death, Simba will lead a friend to the top of a huge mountain looking for something that would save the world, then commit suicide to help his friend survive the trip down. But I don't think that's very Disney-esque.