Training:
I agree, I can't think of any major jutsu Naruto has used that he hasn't been shown training. Even the perfected FRS was shown, sort of, as it showed Naruto sweating in sage mode and sliced trees and mountains. Now Kishi has kept his most recent training under more wraps by simply using Killer Bee to simply say "yeah I taught him that" or "He learned that during my training" when referring to Naruto's new Rasengans or other moves in KM. The only WTF moment that comes to the top of my head is that Naruto can now enter sage mode in 5-10s, so he must've trained in sage mode as well as KM while in the isolation chamber. This eliminates the biggest weakness of sage mode followed by its finite duration. Thus, it's possible Naruto trained on "that jutsu" with Bee or behind Bee's back in KM, but Kishi never showed it, and unless he intends on showing flashbacks, I'd be surprised to see it in this battle, as new jutsu out of nowhere is more how Sasuke operates. I think Naruto's strongest jutsu in this battle will be whiffing on the TBB Rasengan a few times, getting insulted, finding his inner guts, and finally puling it off a few times... standard Naruto tactics, lol.
Tobi fight questions:
1.) The only thing is that all 5 Kage said "Win!" That indicates to me Naruto will win this fight, and it's a common plot device in other manga/anime as well as other writing. However, I'm wondering if Tobi is really the final villain, and "that jutsu" will be used against someone else like Sasuke or Kabuto. Further, how strong is Naruto right now do to his clones? He basically just received an exhausted clone back that was with the Kages I believe, so lets say that one basically started him short 5-10% chakra, where is he gonna be at between 1-1-[90-100]? Basically he got back the exhaustion and no chakra from releasing the jutsu; however, he needs to release the other clones to fight with as much power as he can for Tobi who cannot be held back against. I anticipate the Kyuubi going all in in this fight though, so I guess I'm not as worried about chakra. He won't give Naruto never ending access to the chakra, but for this fight I believe he will or multiple times he'll share his power. I think he'll prefer Naruto over Tobi just as he did over Madara, which isn't shocking since he'll cease to exist if Tobi gets him.
2.) Killer Bee will need to be at his best. He's incredible, but Naruto is on another level as is Tobi especially because of his experience and knowledge. Further, Bee will do anything to protect Naruto (see Naruto vs. Raikage), which sets up your possible Bee capture. This makes sense in some ways as Naruto gets whole, as does Tobi. The story recuperates. Having a surprise fight doesn't seem the way for Tobi and Naruto to settle things. Personally, I think Naruto caught Tobi off guard, and this fight will serve primarily for Naruto to figure out a lot about Tobi and his form of the six paths of Pain jutsu.
Re: Kyuubi:
I'm intrigued as to what the Kyuubi will say about Tobi. Will he recognize him as Madara? I know he shouldn't be Madara as his soul is in Edo Madara on another battlefield, but I wonder if something weird will be revealed through him such as, "This chakra is familiar but there's something wrong with it, I sense Uchiha Madara, but it's not him..."
As I said, I think the Kyuubi will give more and more to Naruto, as he realizes his options are limited. He has Madara and Tobi. Madara treats him horribly, Tobi wants to annihilate him for the Juubi resurrection, and Naruto has promised to never hurt him again. Thus, it's a pretty easy decision. However, I think the Kyuubi has major trust issues so it's still going to be a building process.
Re: TBB & TB II mode:
I don't really want to see the TBB anymore because that means Naruto going into TB II mode, which means looking like the Kyuubi quite possibly and shooting it out of his mouth like he has in his previous cloak modes. I'd rather him master the TBB Rasengan, unless that's what you were referring too, but he doesn't need the fox for that anyway. I hope TB II mode is an evolution of his current KM that makes him more like the Rikudou Sennin/younger son, and/or that fusion with sage mode does that.
Peace
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Going back to the original question does Naruto have a chance? That's insane right? Of course Naruto has a chance, and Minato planned for this 16 years ago, and so far so good with Naruto's development for 16 year old to take on the major villain Minato couldn't finish... His shadow clones that have about 5-10% of his power have been taking out Edo Kages. If Edo Kages and Kage level Shinobi (S-class worth of Kage position) counted on your list of Kages killed, Naruto might lead that list. Sure there's plot no jutsu involved in his fights more in some, less in others, but it's a story not an arcade fighter.
Personally, I wonder if Naruto will be the number one target if when Tobi sees Bee with Naruto depending on how well Naruto fights. If Naruto, with Bee present, becomes too difficult to deal with, I think Tobi will go after Bee and warp them out of there. If all 5 Kages didn't believe in Naruto, they wouldn't of let him fight Tobi, now's another chance for him to surpass his father the fourth Hokage, and I expect to do it, if not in this battle, than in a following one, where this one serves for each side to feel each other out and take conciliatory prizes.
Peace