Kakashi might be battling the Juubi inside Obito.

[MizukiIsTobi]

I actually don't think this is a bad theory as to why Kakashi hasn't returned. I'm not sure how exactly Obito sealed the Juubi, but it kind of looks like he simply absorbed it with his sharingan

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Notice how Obito clutches his sharingan eye in the last panel. I'm not saying it means anything, but it could be a possibility.

 

[Naijakid]

Kakashi will vanish the juubi