>...are you saying he can't NOT be the star of a movie?

Of course not. In fact, he's not the star of Winter's Tale, which he's filming right now. It doesn't look like he's the lead in After Earth either. But I can see why, given the subject matter of this particular movie, he wanted to make sure that he was THE protagonist of the film.

>I think playing Django would have been better than playing J
>again in Men in Black III

And Will Smith disagreed. Will Smith, the man who built himself into one of the top stars, if not the top star, in the world. And after not making a movie for four years, I can see why he opted against potentially pressing reset on his brand, which is what Django definitely could have done.

>This is the same cat who could have been Neo in The Matrix

Leo DiCaprio turned it down as well. Ironically, Will Smith initially turned down Men in Black. Actors turn down roles all the time. Actors ask for rewrites all the time. Will Smith has a ways to go to match Harrison Ford in both of those.