Once a guy has been in the league 13 seasons and a nickname hasn't really stuck, I think it's safe to say that giving him a new one is a losing effort.
I like "Ray Ray", as well. I don't like "Rain Man", because I don't think it makes sense to give him the same nickname as another prominent all-star from a decade ago. That would be like naming some rookie "The Truth". I know Kemp became a joke, but he used to be one of the best players in the NBA, and the "Reign Man" moniker is instantly recognizable to most NBA fans.