Considering what he said about how the feature might not be implemented, it suggests an element of complexity that most haven't mentioned yet. I actually think it might be some sort of "video" mode that allows you to record your Smash battles. Now, hear me out, it wouldn't actually be a video recording--that would be incredibly wasteful and slow down the battle--but it would record player inputs and random events like item drops, like rerecording in PC games and in emulators. This would be both supremely awesome and take an incredible amount of testing to implement, so I think it fits the bill!
While I don't seriously expect the above to happen, it would be an amazing surprise and would add a whole new dimension to Smash play. Pro players would be pretty happy, I'm guessing.
Edit: And when I say "most haven't mentioned yet" I mean "I didn't read far back enough."