Let Rey end on his own terms please. The ultimate underdog of the last generation shouldn't go out with his head low.
If someone hypothetically has to do it my immediate thought is Del Rio but at this point it would do nothing for him, a few years ago maybe but I'm not keen on that feud anymore really.
Alternatively just have him announce on his own terms that he's going to retire and schedule a friendly ("one last") match the next Raw against whoever, some big face, mentor, long time rival, guy he never wrestled, etc. Perhaps John Cena, CM Punk, Daniel Bryan, possibly Sin Cara but probably not (I still want some more interaction between those two though).
Point is, the last thing I want is for Rey's departure to be a downer.