The Yin chakra was most likely from Naruto, Chakra can be made transparent and solid according to how Susano'o works, It might be even something only for the user.
Well, the Kyuubi used a Bijuu Dama when he was fighting Naruto when Naruto was trying to control his power with the help from Bee. If Kyuubi made it himself, he must have Yin chakra. I think this is just a plot hole that Kishi hasn't thought of to much, and doesn't really mean anything. Yes it's been said that Naruto has the Yang and Minato has the Yin, but Kishi I think is just overlooking the fact that you need Yin and Yang to make a Bijuu Dama, and there is no meaning behind it. Or I guess you could say that since Naruto and Kurama were fighting inside of Naruto's Bijuu World or whatever you want to call it, he still had access to Naruto's chakra, and took some from him even while battling.
I doubt it was for the seal, we never had another Perfest Kyuubi jinchuriki so I can prove you either wrong or right.
Well, Minato is kind of a perfect Jin now isn't he? The other half of Kurama was working with him also, that's basically what a perfect Jin is. I don't see why everyone thinks Minato's mode is different than Naruto's anyway, it's the same thing, just darker, because he has the Yin chakra, and Naruto has Yang.
My theory;
When Sage divided Juubi chakra into 9 parts, he then added some of his chakra only into the last bijuu Kurama.
This could explained when he win against Kurama, Naruto gain the cloak similar to Sage, and some of Kurama chakra.
Because Naruto body was small, Minato could only put the 'yang' Kurama.
When the 'yang' Kurama about to be taken by Madara, Kurama told Gaara to go to Minato and transfer the Kurama to Naruto.
This could means how Hagoromo knew what and how Naruto live.
Then finally, Hagoromo decided to 'entrust' somethings to Naruto.
That is why the Toad Sage said about destiny....child....
Eh, I kind of agreed with the sage putting his chakra inside of kurama or the other beasts before, but with this latest chapter, it is pretty clear that the sage just "is". He is just chakra, and he can do whatever he wants because he is that sage, that's how it was put out according to this chapter. I'm sure the sage could talk to anyone he wants to in their own mind like he is doing with Naruto, but he obviously doesn't feel the need, and he only talked to Naruto because Naruto is the reincarnation of his son.
It's a good theory, but I think Ashura's form here has many possibilities. I agree and disagree with you in part. I think he may in fact have been Kurama's first Jinchuriki as the facial features, symbols and appendages are just too uncanny, but that he either mixed his version of the eight gates, which I believe he developed with it or as you said, his own massive yang chakra.
Naruto using the pieces of the bijuus with it was a good thought. Perhaps he will manifest multiple appendages as well, each one representing one of the other eight bijuus and using their unique bijuu damas or other abilities while Naruto and Kurama's power will be centered in the head of the form and their fused chakras will manifest two faces like with Ashura. In a way, the idea sounds similar to when the rookie nine joined with Naruto and Kurama to battle Obito.
Honestly I don't know why everyone is thinking that Ashura's using Bijuu Mode, it doesn't look like Kurama, it does not, absolutely not, have the face of a fox, and the rest of it it just the body of a humanoid, with a lot of arms. Obviously it could look like Kurama because Kurama himself is basically just a human with a fox face and tails, and claws instead of finger nails. It looks more like Buddah or Hindu symbolism to me than it does Kurama, which makes sense, since he is the sages son. Naruto's Bijuu mode has the same markings at least because, well because he IS Ashura.
Assuming ashura wasn't a jinch and that cloak is just his own chakra, then Naruto won't be able to attain that power, which is why he's a jinchuriki. Ashura is that powerful because he's the son of the sage, where as Naruto is the son of an uzumaki and namikaze, the bloodline is too diluted for him to be as strong as ashura is without any help from the bijuu.
Well, you are kind of wrong there. Naruto technically IS Ashura, so I am pretty sure he is going to be that powerful. Ashura's chakra is reincarnated inside of Naruto, sooo that basically means Naruto is going to have the same beast chakra as Ashura. Well he already has beast chakra, but it will be even more so. The sage didn't say Naruto is the reincarnation of his son for nothing.