You'll have to forgive late night musing, but this time it turned up something rather productive and surprisingly plausible since I was staring at the ceiling in the middle of the night.
Everyone knows that Kopa is technically a sub-canon character, and according to that sub-canon, is Simba's and Nala's son. Due to the time constraints and ages of the canon characters, both before and during the sequel movie, it makes that plausibility a little hard to squeeze in properly. This theory (which I am using in my "Paw Prints in the Stars" chronology of the series) actually makes things entirely possible if you look at another sub-canon character: Ni.
Being a near Simba-expy in appearance, it is entirely possible that Ni is Kopa's father. Being the harsh reality of nature, both he and his mate could've died from some thing or another. One of the two could've delivered the cub to Pride Rock, quite possibly on their last breath. Considering the nature of Nala and Simba, it's entirely possible that they had taken in the cub and raised him as their own. While the problem of Nala not being able to nurse him is there, it is possible another lioness was nursing her own cubs and provided surrogate milk.
And thus, it all ties in with Zira's existence for his disappearance. Whether you believe he died or is simply lost, my estimate is that she led him astray between four and five months of age while being freshly pregnant from a rogue herself. (So to say, following Scar's/Taka's orders that her second-born son would become King, and thus getting herself exiled in attempt to carry out her dead lover's will.)
However, this is just my two cents' worth. Some finer points still need to be ironed out, but this is the rough idea.