I would think Palpatine, personally. The first Walgreens exclusive was a variation on a previously released figure: Boba Fett. Considering that Palpatine is on the way in the 6" basic figure line I can kind of understand an exclusive Force Lightning version for Walgreens. Especially in light of the fact that the Walgreens toy buyer tweeted an image of lightning when someone asked him a direct question about the 2015 exclusive.
I guess what made me think of Vader is that he is the one that does the Force Choking. Luke is the only other person in the movies that does it and Force Choke was the hint that the Hasbro rep made (not Jeff L). Vader would be a variant, too, since Vader has also been released and the could add the lightning, see-through helmet and amputated wrist. But since I listed the different parts, I can see why it probably wouldn't be Vader since Fett was a re-paint. The Emperor would be cheaper since there wouldn't be retooling involved (just adding some lightning in the package). But that Vader would be cool and welcomed by me.
The only thing about the exclusive Emperor is that if the only difference is the inclusion of Force Lightning, I would most likely wait for that version and skip the regular retail release... and I'm sure I'm not the only one with such sentiments.