Alright, you want relevance.

Eminem and Pac would probably not be down with eachother, given that Pac was a Black Panther and though he had many white fans, he talked about the black man struggle and the white man burden on the black man.

However, since he has many white fans, that could be the main reason for a collaboration with Eminem, because if Pac was with a white rapper and showed that he was down with everybody, than that may decrease people thinking he was strictly for his niggaz......thus drawing in a larger (if not already large enough) fanbase.

But, what would also bring in sales would be if Pac and Eminem had beefs between eachother, what better battle than Hip-Hop's best black rapper vs. Hip-Hop's best white rapper (public thoughts at least)?
People talk day in and day out about these mediocre Ja Rule/ 50 Cent beefs, so this Pac and Eminem thing would have been huge.

But personally, I don't think they would be down with eachother on a personal, chill somewhere on the weekends level. But on a business level, definitely.