Naruto isn't going to get water style. This is like all those other threads we've seen over the years with "wouldn't it be cool if..." and then mentions things that range from the absurd to the mostly-reasonable but upsupported by the manga. Wouldn't it be cool if Naruto got byakugan? Wouldn't it be cool if Naruto unlocked his own Rinnegan? Wouldn't it be cool if Naruto learned all the most powerful Uzumaki sealing techniques? Wouldn't it be cool if he unlocked his own wood style? Wouldn't it be cool if he got water style too?
Yes, it would be kind of cool in a way. Yes, it would bring a certain amount of symmetry to the final battle between naruto and sasuke. However, this never was a symmetrical battle and they are not symmetrical characters.
You have to look at the two in terms of the characters, and what Kishi is trying to show through each of them. It's not a matter of these two characters having complimentary lists of moves, where each of Naruto's offensive moves is perfectly countered by each of Sasuke's defensive moves and vice-versa. The difference between the two characters goes far deeper than that. They are two characters of the same age, from the same village, with deep personal hardships to overcome. But they way they've gotten where they are now is completely different:
Naruto has never really had anything. He had no family. In school he had few friends. Sakura never loved him. His teachers didn't like or respect him. He couldn't use any jutsu. He couldn't win practice fights. Everybody thought he was a loser or worse. Because of the fox inside him, many people saw him as being a monster.
Sasuke on the other hand had a loving family. A loving mother, a caring brother, and a father who was a little gruff but well-respected and who demanded excellence. Yes, they did die, but Sasuke did have those experiences first, and then felt the pain of loss. Sasuke was cool. The girls all loved him, the rest of the class all respected him, and his teachers had high praise for him.
Naruto went on to overcome adversity. He learned to control his chakra better, even though Orochimaru's seal made his chakra control erratic, and the fox was always threatening to take over his mind and turn him into an uncontrollable beast. Later, Naruto was told not to use the fox's chakra at all, because it was so dangerous, despite being also his largest source of power.
Sasuke didn't have any problems. His sharingan made him an almost immediate master of taijutsu. He was able to learn new jutsu very easily. In a month he was able to copy and replicate Lee's taijutsu skills AND his speed (though not his stamina). He was able to learn chidori quickly, and then use it in a very tricky battle.
Sasuke's ability to learn things quickly, and his insatiable thirst for power and revenge have allowed him to accrue many things: summons, seals, fire and lighting techniques, skills with his sword. His new MS and then EMS eyes. Whatever medical and training enhancements Orochimaru gave him over the years. Naruto, on the other hand, has to fight for everything he has gotten. Naruto learns things the hard way, and as a result he has far fewer techniques in his arsenal. To compensate, Naruto has to cleverly master and combine his few techniques in interesting and unpredicted ways.
Sasuke is quantity, Naruto is quality. Sasuke is breadth, Naruto is depth. Sasuke had family and lost it, Naruto had nothing and gained friends and family. Sasuke is hatred and revenge, Naruto is love and protecting the people he loves. Sasuke has mastered two elements and uses them in various ways, Naruto has exactly one technique that uses exactly one element, and he barely understands how he does that much.
Naruto does better not by learning new things, but by gaining a deeper mastery over the techniques he already has. Not by gaining new eyes (like Sasuke), but by deepening his existing bond with the fox. Not by learning new elements (like Sasuke), but by knowing when to and when not to use rasenshuriken for great effect. Not by picking up multiple summons (snakes first, then hawks), but by befriending and strengthening his friendships with just one type of summon (toads).
The only technique I could possibly see Naruto trying to learn at this point is FTG because it's a legacy from his father. But even that seems a little bit unlikely this far into the war. The only way he would possibly need that is if Tobi gets some kind of massive, overwhelming powerup that can't be countered in any other way. I won't hold my breath over it, however.
If you look
