I think a straight pardon is unlikely for the next 10 years, unless something on the order of the Church Committee happens, if not more. (IIRC you have to be convicted to be pardoned, anyway)

However, I could see some kind of immunity from prosecution granted to get him out of Russia, and to confirm what documents were released, happening in a somewhat more marginal change (maybe at the next Presidential transition, but probably not). This would be lesser, because it wouldn't be saying "what you did was right", just that it's in the national interest to end the whole affair.