Uzamaki Have an Affinity for Natural Energy
Here I will make a case that the true bloodline limit of the Uzamaki Clan is an affinity for Natural Energy. In the sense that their bodies can take in and control natural energy better than anyone else in the verse.
Example 1: Jinchuuriki
Chapter 610 tells us Juubi is comprised of Natural Energy:
Kura: that thing… generally gives the same sensations of the earth, the water… and the air… it has the same natural energy of the things moving around in this world\\
kura: things become different when you go into sennin mode and use the same energy
Thus it stands to reason that the other Bijuu's, who were once part of Juubi, powers also come from natural energy. Now what happens typically when a Jinchuuriki draws in the power of a Bijuu; he/she takes on the appearance of that Bijuu until eventually they become that Bijuu and if their bodies can't handle it they even die.
Way back in the manga we even got talk of compatibility between the Bijuu and the host. Garaa was deemed incompatible as a host of Shukaku, which is why Yondaime Kazekage and the Sand Village viewed him as such a threat and even tried to assassinate him.
Now I ask you what happens when someone is trying to master Senjutsu and their body can't handle it. The natural energy takes them over and they slowly become (Toad, Snake, etc..) & if there body truly can't handle it they die.
Now if we accept that the process of Jinchuuriki drawing on Bijuu Chakra is the same as a Sage drawing on natural energy, except they can move while doing so since the Natural energy is already gather than all of this fits together nicely.
But than let me ask you why did Hashirama choose an Uzamaki to be the container of Kurama, the village choose Kushina, & Minato choose Naruto? It stands to reason that these exceptional Uzamaki were always chosen because they were especially adept at handling Natural Energy. This also fits with the fact that the Uzamaki are decedents of the Sage who would have also had to have had this affinity for natural energy to house the Juubi. In-fact many have speculate that the Sage wasn't a Senju, but the original Uzamaki.
Finally this would explain why Naruto's transformations when entering BM/RM are so different from the typical V2 & Full Bijuu Transformations that other Jinchuuriki posses, because Naruto not only was trained in Senjutsu by Fusaku, but he also possesses the natural Uzamaki affinity for Natural Energy.
Example 2: Uzamaki Naruto
Back in the Pain Arc many people called BS at how fast Naruto managed to master Senjutsu. Heck even legendary Sannin like Orochimaru & Jiraiya could only master Senjutsu to a certain extent even with years of experience, but Naruto comes along and masters is far beyond Orochimaru's capabilities & even J-man's in the matter of a week! Seems unrealistic, heck even I thought this was typical Shounen Hero Plot Kai. However if Uzamaki have a natural affinity to Senjutsu this would suddenly explain Naruto's insanely quick mastery of Senjutsu.
Example 3: Yakushi Kabuto
Let's remember even Orochimaru could not master Senjutsu, so Kabuto gaining Orochimaru's power shouldn't have allowed him to do this & if he could have done so with his own power before, than why didn't he? However what would explain Kabuto ability to master Senjutsu when Orochimaru couldn't is the fact that he took in Karin's Uzamaki DNA. Again if this DNA gave Kabuto the affinity for Natural Energy this would explain why Kabuto was able to master Senjutsu in such a short time after Orochimaru's death.
Why Does This Matter?
Because it opens up a plethora of possibilities. Rikudo being the original Uzamkai is one, which was already addressed, but perhaps more interesting possibilities involve Naruto. So far Naruto has only been using Kurama's power as a normal Jinchuuriki would, which makes sense because Killer B was his teacher and he's only a normal Jinchuuriki.
However if this idea is right and Naruto as an Uzamaki can manipulate the natural energy that Kurama is made up of to a much higher degree than normal Jinchuuriki, than he should be able to also use Sage Techniques. In-fact we already have examples of this via Naruto being able to use FRS in KCM Mode. This means Naruto has not even begun to utilize BM/KCM to their full potential.
Remember Kabuto's Muki Tensei imagine that used with a source of natural energy like Kurama. Remember Frog Fu, well imagine those Ghost punches being used with that level of natural energy. Finally let's remember that Naruto himself can draw in Natural Energy, so he can keep drawing in natural energy to restore or even increase Kurama's natural energy supply. All of suddenly match Juubi in raw power doesn't seem so far fetch for this duo.
Finally we have to consider the implications this has on "that jutsu". It would most likely be some type of ultimate Senjutsu technique.
Naruto could master utilizing Kurama's energies this way in this battle or more likely this will be the final training arc for Naruto.
I would also think that there is potentially some connection between Sharingan & Natural energy, & their eyes lacking the Uzamaki affinity for natural energy can't handle it and that's why they become sick/blind/etc... and have to steal further ocular power or DNA related to the Sage to avoid this. But I haven't thought through all of those aspects yet.