Besides, if Naruto is really the reincarnation of Rikudo Sennin, he cannot be any of the Uchiha or Senju alone. Rather, he had to be a uniting factor of both clan - that is he has to have the two bloodline. Thus, as my understanding goes, the only way for Naruto to stop the mad competition between Senju and Uchiha is to have both bloodline - that is to have the bloodline of Rikudo himself.
If the reincarnation theory is true, then yes, I agree. I don't think that Naruto has to be the next Rikudou to stop that. Sasuke plays this part maybe even more than Naruto since his side is the one who always picks the fight in each generation.
Thus, if we actually consider the fact that Rikudo gave his heritage to the younger son because the younger son had his will, then the Uchiha will have to fight anyone who inherits the will of Rikudo. Does this sound that only the Senju can inherit the will of Rikudo?
I doubt it. Even Itachi Uchia had the will of fire. This is the reason he almost annihilated the Uchiha clan. Uchiha Shisui, Uchiha Kagami and others inherited the Will of fire. Even Sarutobi inherited the same Will. Why those non-Senju Inherited the Will of Fire? The answer is simple. The will of fire has become a school of thought that any Shinobi in Konha can inherit from the teaching of the Hokage. Clearly, Naruto, Minato, Kushina, and others such as Shikamru or Assuma all inherited the same will as taught by Sarutobi to all the generations of Ninja he saw. Thus, having the will of fire and fighting an Uchiha does not make someone a Senju.
In addition, the idea that Naruto is the body and Sasuke is the eyes is clearly a fan-made idea. Of course, it was enforce by Obito's machination mainly used to fuel hatred between Naruto who was his main problem and Sasuke who he wanted to use as a mean to get the Kyuubi. Otherwise, there are many signs in the manga that related Naruto more to Rikudo than to a Senju. These are the obvious one:
(1) Rikudo was born from someone who had the chakra of Shinju/Juubi. So was Naruto born from a woman who had Kyuubi, the largest peace of Juubi inside her. Basically, we could say that Naruto was born from a woman who had the chakra of Shinju inside her because Kyuubi is a mass of chakra.
(2) Rikudo beat Juubi and sealed inside him. Naruto, as soon as he was born, became the only option to stop Kyuubi's rampage.
(3) Rikudo was a sage, and I am pretty sure he was connected to the toads. Naruto is a sage at a younger age, and connected to the toads.
(4) The sage created the Bijus, and gave them a mission to find someone who will do better than he did. The Bijus acknowledge Naruto as the person the sage predicted.
These are clear signs that show that Naruto has been depicted more as someone with the same feats as Rikudo than someone from the Senju clan. In fact, I predicted long ago that unless Kishi find a good reason for Naruto vs Sasuke, it will be the lamest fight ever given in the manga. Currently, the manga as nothing to do with Uchiha vs Senju. Rather, it is about people fighting to accomplish Rikudo's ultimate goal that is to bring peace in the world. Madara, Obito, and Orochimaru - all the main villain - were researching to attain Rikudo's power. I believe that the only thing that can make sense is that Naruto will be the one to achieve this power as Rikudo had or better than him.
Why would that be a fan-made idea? There is a lot of sense in what Obito told Naruto, especially when you see how much he and Sasuke resemble Hashirama and Madara, not to mention the first sons. That, on the other hand, doesn't undermine your theory on him becoming the next Rikudou. In the beginning, his power was practicaly split in two and was deteriorated through generations as time passed by but it is understandable that there would be a descendant who'd attain the same or even more power than the grand father eventually.
I agree on the lameness of Naruto and Sasuke fight. That should've been a dead end as soon as we learned that Itachi inherited the will of fire more than anyone else. Naruto probably will get to be like Rikudou, but he won't become him himself, he'd have similar power, which doesn't mean he can't be of Senju heritage.
Madara was never frustrated because he was weaker than Hashirama, his anger and actions were due to his concern for his brother and the Uchiha. Sasuke was probably the only one who felt frustrated at "being weak," and that's because he underestimated Naruto and he was obsessed with power in order to kill Itachi. I don't think the Uchiha inherited the Rikudou SEnnin's bad side at all. More like the elder son's rage, which he may have passed down to his generation.
I honestly find it funny how some will think Naruto will become like or the next Rikudou Sennin. Maybe in ideology only, but he's nowhere strong enough for that. If Naruto can't even touch Madara, he has no chance of being the next Rikudou Sennin. The only thing that they would have in common is the desire to establish peace.
That certainly weren't the only reasons. Of course he was also pissed because he was weaker. He didn't get EMS out of sheer boredom, he got it because he needed power.
As of this moment, we don't know how Naruto or Sasuke would fare against Madara or Hashirama. But if they beat Obito just now (even with cheesy friendship shit), they certainly aren't easy opponents for them.
Sasuke got EMS because he wasn't at full power, given he was nearly blind. I was talking about Sasuke in Part I, as he was jealous and frustrated at Naruto's growth. That didn't exist in Part II, Sasuke didn't care about Naruto's power or growth since he was strong enough to take on Itachi, in his opinion.
Naruto wasn't able to do anything to Rinnegan Madara though. I also doubt Sasuke could do anything to Rinnegan Madara or Tobi either, given it took three guys to be on par with Tobi. Given what we've seen there is a very remote chance (IMO) that they could beat Hashirama, but I highly doubt they could do anything to Rikudou Sennin. At this point though, why do both or either need to be that powerful? Once they beat Tobi and the Edo Tensei disappear, who else is able to challenge them, as long as the Juubi or bijuu cease to exist or be accessible? Among the current living, Sasuke and Naruto are arguably the most powerful, with Raikage probably being the only one to give them challenge because of his speed.