i really enjoyed your post. bummer that so many people went negative with their comments. i think your proposal is interesting and possible if naruto ever gets around to learning a second nature element (if in fact someone believes he hasn't learned how to control the other elements already).
despite all the things he has learned (that none of his peers have) like sage mode, bijuu mode, frog kumata, creating and combining different kinds of chakra, chakra control on the level of rasen techniques, his summoning techniques are on a sennin level (not little dogs, turtles, or hawks (that haven't been shown to do much but fly)), and more...sheesh, naruto can do a lot...but he has not demonstrated ability to do some things that his peers do easily like being able to perform more than one nature technique (kakashi can use all of them), sealing techniques (and he comes from a family known for their sealing techniques as others pointed out), medical ninjutsu (though his body has a self-healing ability, he can't heal anyone else. sakura, ino, and hinata all have medical ninjitsu), we don't even see a lot of variety in what he does (shadow clones, rasengan, tai jitsu/frog kumata, rasenshuriken...that battle sound familiar?).
i'd like to see him use some of the wind techniques of the sand ninja, something like that air cutting technique used to cut the legs of the summoning beast, or some of danzo's techniques. i might argue he hasn't really mastered air just yet. what about that second element, to your point? though maybe he should learn fire. he is from the leaf (will of fire and all) with the sarutobi clan and a history of uchiha. bijuu mode also puts him on fire sorta. though maybe it doesn't burn anyone. kakashi and jiriyia, two of three sensies, can both use fire techniques. weird that he learned nothing from them. and without air (something he can control already), you can't make fire so shouldn't they be closely related? conversely, isn't the ability to use fire a demonstration of also being able to control air to some degree? i have a yet to see a fire attack that was not controlled directionally in the air. if you believe that argument, why not lightening? no air, no lightening. that's why there are no lightening storms on the moon (no atmosphere aka air to speak of), but you do hear about atmospheric storms on venus and saturn. plus kakashi (his sensei) taught sasuke (his teammate and rival), sasuke's second nature element (fire was his first), which just happened to be lightening. maybe he should learn earth next? he can already use nature chakra by taking energy from the ground and living things. shouldn't that be a close cousin of having the ability to use chakra to move or create the ground rather than just taking the ground's energy? point is, he already has a connection to the earth in sage mode. and his sensing ability may be somewhat earth related, though admittedly not entirely. in summary, i think someone (not me) could argue that water is the only element he doesn't have much of a connection to other than genealogy and the only one we've never seen him use or affect in some way (aside from being able to stand on it). and who knows maybe wind from his dad trumps water from his mom. a brown eye, blue eye thing.
what about some weapons? his dad used a kunai (granted it may be more related to the 4th's space time jitsu than the power of kunai itself. the 3rd had that staff.). sasuke even uses a sword in most battles now. genjitsu?!? any uchiha, that guy from the land of lightening during the fight with sasuke v raikage, nagato (though dead now), the chick with red eyes that is baby mama to asuma, orochimaru (he was dead, but now alive again, but never was really dead to start with?), etc.
and what about the things he started to learn but we don't see anymore? they made a big deal out of him learning to combine his attacks with frog summonings, but we've seen just one and that seems like forever ago. he summons but fights independent of their abilities (and why hasn't he summoned any for this final battle we're seeing now? if i thought the world was coming to an end, i would summon everyone i could.).
if you consider the genealogy argument that he is connected to the water element via his mother and name, with my arguments for how he is also already showing an affinity for fire and earth elements, has air element and techniques, arguably fire techniques via kurama, and without air there can be no lightening, you start to see a picture of him already having a predisposition to all 5 elements. and don't the most powerful demonstrate the ability to control all of them? sage of 6 paths, kakashi, the 3rd allegedly, and you even say in your post you assume a hokage could control most, if not all (have we seen the 5th use any element techniques?). one could argue he has a "relationship" with every element already...maybe lightening is a stretch, but he could easily learn it from kakashi who can obviously successfully teach it (sasuke).
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what was the subject of this post again? point is, i enjoyed it. maybe the question should be what should his second nature technique be? it's really his ability to performs techniques of different nature element that is at question, one could argue. ah, semantics. i assume you have to have a connection to the element to be able to use the element's technique. or is it chicken before the egg? my vote for second technique to learn (if you don't believe kurama should count as ability to use fire techniques) would be earth techniques style. supposed to be so powerful, but we barely get to see them since only 2 people have the ability to use that techniques. but maybe you have to master earth and water first? or maybe you just have to have a relationship with those elements, which naruto already has? lol.
