Naruto goes insane (no, I am not joking). He loses his sole purpose to go on. Remember, he drew a lot of strength from the fact that Sasuke was alive somewhere and he was going to get him back. His unconditional love for Sasuke drove him to great lengths; every technique he learned, every step he took to befriend Kurama, even willing sacrifice he made on his own part ... was for Sasuke and no one else. Sasuke is the foil for his entire psychological and emotional development. He made Naruto strong, or the belief in acquiring that goal made him persevere and endure.
He didn't let Kakashi kill him, albeit he knew he was going to kill his Sensai and Sakura (a girl he loved back then). So, Naruto was willing to sacrifice a lot, even his Self for Sasuke. He was the core motivation for his emotional stability.
Take that out and you have a young man who has lost the one goal, the sole motivation and the lone psychological anchor that is holding him together. That incident at the bridge against Orochimaru is a telling tale. He completely lost control, when his friends were still around. He quickly used to get unhinged and deranged, allowing Kurama to make the crack larger in the seal. Imagine if he finally finds out the Sasuke is gone for good, and the only way to get him back is to use Rinne-Tensai via Nagato and become the Sage of the Six Paths baser foil, Jubi Jin.
He would surely take that leap, and the person who would aid him would be Itachi. Itachi loves no one more than Sasuke. He slaughtered his entire clan for Sasuke and didn't even show mercy to his own parents, who loved him far more than their younger son, whom they always neglected. Kabuto's ET hold was broken by KA, which issued a 'protect Konoha' command on Itachi. He should have killed Sasuke (despite not desiring to) when he blatantly declared that he would destroy Konoha.
He overcame that command and didn't care much for the village. That says a lot about the depth of Itachi's love, albeit it was severely warped and twisted. Hence, if Sasuke is killed, Itachi would leak Konoha's secret and would attack Konoha first with Akatsuki. With Sasuke out of the picture, he has no reason to protect Konoha as he was there in the village to threaten Danzo in the past. He also didn't stop other Jins abductions and assassinations, which would further dampen his spirits to keep the ruse going.
He would integrate himself well into the plan, and will either 1) abduct Naruto or 2) seek out his aid. I can defninitely see Itachi trapping himself in IT to see Sasuke or using Rinnegan's Rinne Tensai to bring him back. I cannot see Itachi and Naruto sitting still, and I can definitely see Naruto's insanity being a very definite end for his character.