As we know, through sheer Fuinjutsu skill Minato achieved what appears to be an either BM or KCM form. However, I believe it is neither, but something inbetween.
Let's take a look at it.
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That's Minato's Kyuubi cloak.
Now let's compare it to KCM and BM.
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similarities with KCM:
- The color pattern
- THe magatama '' necklace '' on his upper torso
- the pattern of the seal and the black line on his torso
Interesting is that whereas Naruto's seal is on his stomach Minato's is on his chest.
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similarities with Bijuu Mode:
- Two strands of hair spike up, creating the image of horns. Rikudou Sennin and the Child of the Prophecy, anyone?
- You can see chakra ''flying'' from him, just like BM Naruto, as if he is burning.
- The sygils on his hand and collar. KCM does not have those, BM does.
- He has a robe, though that may simply be his.
Arguments suggesting he has a mixture of both, leaning towards BM:
- Naruto managed to take Kurama's chakra to form KCM through sheer will and Kushina's help alone. Not only did Minato seal half of the Kyuubi inside him, he is an edo and as such cannot die. He cannot be overpowered by the Kyuubi and as such can take all of its chakra without worry.
While alive, sealing the Kyuubi in him made him go numb. However, he is dead. He feels no pain. As such, he controls KCM/BM perfectly.
- Minato's sealing abilities far, far, far exceed Naruto's. Through the manipulation of the seal, absent any contact with the actual Tailed Beast, Minato managed to form KCM. This can only suggest that he would be able to take it further.
- The spikes on his head are similar to BM Naruto's and...
surprise, surprise..
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The Sage's.
Now the whole '' Child of the Prophecy '' / '' Savior of the world '' makes sense.
Naruto, Nagato, Minato.
They form a triangle, because they all have something from the Sage.
Nagato has the Sage's body and the Sage's Rinnegan.
Naruto has the Sage's body, the chakra of the bijuus and the Bijuus as his friend.
Minato.. well, Minato has a KCM/BM which he mastered -by himself- while being -locked inside the belly of the death god-. Such a feat is extraordinary, seeing as how Naruto needed Kurama's aid to reach BM, whereas Minato reached a bastardised version of it.
Bottom line, I believe Minato reached some sort of pseudo Bijuu Mode. IT is stronger than KCM, but weaker than BM.