How about we just skip to the endgame, hmm?
Concerning the box, and what might be inside of it...
Well, what exactly did you expect when John Doe (Kevin Spacey) showed up in the Police Station, covered in blood? He had two more sins to cover, after all.
Sure, it was fun listening to Brad Pitt give his little "do you realize you're insane" speech, but the instant the killer shows up, the whole game turns from a static affair to a deeply personal one. You know that all three of these people probably stand to lose something from whatever denouement Doe has in mind.
Yup. John Doe has the upper hand, indeed.
The entire final scene plays out perfectly - of course, by "perfectly", I mean gut churning. The finale had me staring at the credits straight on through in shock. That's usually the sign of a good ending.
Let's see just what's in that box...