If Naruto merges his body with Sasuke (this has been done many time by many characters such as Kiba and Akamaru, Jiraiya and Ma nd Pa) it'll be d fusion of d senju and uchiha into a single body once again. This would likely bring about a version of d sage, with d ultimate senju's body (Bijuu Mode nd Uzumaki Body) and d Ultimate uchiha's eyes (Rinnegan). The rinnegan will be achieved, as a result of d fusion of d uchiha and senju.
I believe dat Kishi would rather end d series dis way, dan havin Sasuke die, 4 naruto to achieve d rinnegan
Besides, he would only need d rinnegan for a while (while fighting madara) so, he doesn't need it permanently.
Besides d younger son of d sage, is not supposed to be obsessed with power, so naruto should not use d rinnegan permanently, with is a symbol of power through hate.
My Point exactly is that Naruto and Sasuke are the fulfillment of the sage of six paths