surely itachi ment something when he said i hope you never use it! and surely kishi likes to twist things!, but the thing is already twisted, with itachi saying what he said, so what we gotta do is untwist what is twisted by looking at the japanese mythology, and some famous mangas like DBZ and olso looking at and analyzing kishis way of twisting!!since we know what the twist is related to, we only have to analyze the details and guess what the twist will be!!
so, my point is, its much probable that the gift itachi gave to naruto is related to that jeweled necklace from the mythology!, as it has been said many times before sasuke is in the "darkness", the word "darkness" have specifically been used many times in the manga regarding sasuke!, and the word "light", and the oposite of sasuke has been used to describe naruto!!, narutos greatest ability is to see the "light" inside peoples hearts, however small it is, naruto finds it and nourishes it to a point where the darkened heart is no more, and the light is completely restored!, as he did with Nagato!
sasuke on the other hand is abit different, his becoming more and more dark, as he started to kill samurai, karin felt sasukes terrifying darkness, and as he tried to kill karin, he came alittle closer to totall darkness!, and as naruto decided to fight him, "meaning there is no light left in sasukes heart", Sasuke was in totall darkness!!
so how do you light up a heart where there is no light in?, the answer is simple....
you cant!
but, as i always say about my view on the possible and the impossible, a completely darkened heart and soul as sasukes where there is no light left is impossible to light up, but as i say....
"When Something Is Impossible, All You Have To Do Is Convert It From Im-Possible To "Possible!!"
just as it was impossible that from "Nothingness" the "Universe" came to be!!....
The Impossible Became Possible!!
so in short, if it is impossible for naruto to lighten up sasukes heart since he has no light left in his soul, All he has to do is "To Put Back The Light In Him", then the impossible becomes possible, and naruto will be able to lighten up his heart again with his special speeches that he did to nagato and others!!, so how to put back the light in sasuke?, and here comes....
Itachis Gift!!, itachis gift, the jeweled necklace, and the third great item following the yata mirror and the totsuka sword, the yasakani jewel will "Restore The Light Into Sasukes Soul"!!
and as it does, naruto will be able to use his mojo to bring the light back into sasukes heart! "and by that i mean, naruto uses his righteous speeches to make sasuke aware of the light and the good in him!!
and the reason why itachi says "i hope you never have use it" is because he doesnt wish sasuke to be totally consumed by darkness to the point of no return!!, so in case that happened, itachi knew that naruto wouldnt be able to lighten up sasukes heart, so he gave him the only tool that can restore he light in sasukes soul!, but ofcourse it would not be enough without narutos great ability to make people realize the light and the good inside them!!