Why susanoo? Why kishi?

[Alastor]

Pretty sure it is supposed to be a reference to the other portion of the Juubi : Gedo Mazu. That's why Sasuke used it as a substitute for one in the first place.

 

[Holy God]

As per natural evolution there exists diversity. Kaguya cannot replicate all techniques nor is she the absolute "source", as with new mutations comes new kekkei genkai. The only connection Sasuke has to have is with Indra.