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We will never find out because there would be no need for it. Kakashi's raikiri already has shape and nature manipulation so I imagine that if he wanted to he could turn it into a sphere like rasengan but its the most potent when its in the form of a "power surge" as he put it.
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Erm....we already kinda saw what it would look like....
Kakashi made a rasengan, except it was small and blackish looking. He couldn't evolve it any further than that specifically because the nature and form of the chakra just weren't compatible.
