Standard ** measuring between the US and Russia for one.

If Russia ever drops the corruption and becomes a strong democracy, then it could conceivably go into NATO. It would require the US to be much much more passive and a team player (as an American I view this as a good thing, provided it is by other democracies w/ good human rights)

It would also require Russia to have some reason for doing so, mainly if it couldn't defend against China, or some developed Central Asian or Islamist superpower near it. Its also likely that Russia would ally with these forces instead of NATO, or just play them off each other like India is doing with the US/Russia now.

On the NATO side, it would require additional defense commitments by NATO towards Russia (and in the Pacific, the US). I doubt there would be appetite for it in the EU (and maybe not in the US), unless something went really really bad.

Russia has already asked to join nato in the past