It could be just a general power boost for Naruto when he finally gets the resolve to kill Sasuke. I mean, that could clear up the "I hope you never have to use it" statement since he wants to bring Sasuke back. Itachi did say "power" right?
My guess right now is that it is something to counter the Sharingan, right now that is by far the strongest weapon at Sasuke's disposal. Itachi understands Sharingan very well and quite likely developed abilities to counter it, since he had to face it when slaughtering the clan and likely originally intended to fight Madara as well at some point. Itachi also knows about the Sharingan's ability to suppress and control the nine-tails. In order to beat Sasuke, Naruto needs something to counter the Sharingan. It would make a lot of sense for Itachi to have thought of the possibility of this scenario and developed a solution.