[quote="WildSimba"]My theory is that Nala's father eigther abandoned her mother, or died. But considering he's got the same basic light color of skin that Simba has, maybe she's Mufasa's daughter?[/quote]
I like your first thought there.

But Mufasa being her father. I don't think that's possible... First of all due to the fact that Simba and Nala would be step siblings,that would be an insult to the movie itself.
Mufasa can't be her father becuase of many reasons. In the movie he had no interaction at all neither to Nala nor to Sarafina.In the scene where they sleep,Mufasa,Sarabi and Simba are on the middle rock,as they represent the king,queen and prince,if Nala was his daughter,wouldn't she be up there too...
Simba says that Nala is his friend,but if Mufasa was her father he would have to say sister,right?
Zazu tells how it's a tradition going back ages that they will get married( just like in the royal families),and that means that it would be impossible for them to be step siblings because that would mean that pride rock lions were incestious for generations.Now how little sense would that make!
When Mufasa saves them from the hyenas,he says:'If you ever come near my SON again...' if Nala was his daughter,he would probably mention them as his children.
Later on when Mufasa is talking to Simba alone,he says that he is dissapointed in him because he didn't listen to him and further more becuase he put Nala in danger-which is normal for him to worry becuase for a parent taking care of someone else's kid is a greater responsibility than to take care of your own.