There are already three characters with the Rinnegan, giving it to Kakashi would be silly. Giving it to Sasuke would be weird but sorta understandable, Kakashi just doesn't fit in here. Madara unlocked it because he had some Senju DNA available to him plus EMS, Tobi had a Sharingan and stole a Rinnegan, Nagato seems to have been born with the Rinnegan. So I don't see how Kakashi could obtain it, he doesn't have the means to achieve this.
When it comes to Sasuke (i've heard the theory), if Tobi were to pull some crazy weird trick to get Sasuke to unlock it by giving him some Senju DNA, then he could indeed end up like Madara, albeit a bit unstable. I wouldn't really like this, but its not completely impossible.
Point is, I can see no concieveable way that Kakashi could unlock the Rinnegan.