Sasuke's upcoming powerup
I believe Kabuto to be the man showing up, some points in that favor:
The mystery mans silhouette looks somewhat similar to DSM Kabuto's chakra. It's not quite as thick, nor is the color the same, but I expect these to be byproducts of Orochimaru taking the power of the White Snake back from Kabuto. [x]
Karin states that the chakra gives her a bad feeling, most likely associated with the darker methods of the Ryuchi Cave in creating Sage Transformation, or Sage Mode as Kabuto likes to call it. [x] [x]
All methods of Sage Transformation/Sage Mode achieved through the Ryuchi Cave are described with malice. Keep in mind for later: The method of the Ryuchi Cave is not the same as Mount Myoboku, and does affect the users chakra with darkness and coldness.
From this moment on Karin has no intelligence nor connection to Kabuto, so her failing to recognize him after the many changes he has produced on his body since that time is nothing unsurprising.
Therefore, Karin's lack of recognition is not a proper argument against Kabuto's involvement. He still remains the only logical choice to be at that location as well having interest in Sasuke, now with a likely separate philosophy after escaping Izanami, however.
After Izanami's activation, it would make some sense with this revelation that he would help Sasuke, and have a similar change of heart as Orochimaru.
Sage Transformation/Dragon Sage Mode Sasuke
I believe Sasuke's survival will hinge on the mass of Juugo's flesh and chakra which lies dormant within his chest.
The entirety of Sasuke's chest and much of his neck is made up of cells from Juugo, the wound is right upon where the cells are situated.
Kabuto, who if you got the gist from some of the explanations above, knows the method needed to activate Juugo's cells within Sasuke, as he did so in himself in achieving DSM which is basically a controlled variant of Sage Transformation. Using the healing abilities associated with Sage Tranformation Sasuke will be saved. But will he utilize Sage Transformation or Dragon Sage Mode?
This states rather bluntly that he has that capability, I have some other ideas on this however that I might expand later, but the gist is that Sasuke has pretty much all the required components to become a Dragon Sage, with some interference form Kabuto.