Back in 2002, R. Kelly dropped Best of Both Worlds, an album length collaboration that put him side by side with Jay-Z. The album represented a collision between one of the top performers, songwriters and producers in R&B with the most dominant MC in the game (Jay was coming off the success of The Blueprint, a true turning point in his career). Though it was hotly anticipated, the album was considered a bit of a disappointment, if only because of the high level of talent involved. And even though there was a second volume of collaborations (2004's Unfinished Business), their collaboration never ended up being as big as it probably should have.
As it turns out, the tag-team with Jay-Z wasn't necessarily meant to be. In an interview with XXL magazine, Kels admitted that the Best of Both Worlds concept was originally meant to be a collaboration between himself and Tupac.
"I said 'Dude, we need to do an album.' See, Best Of Both Worlds was for me and Pac at first," Kelly explained. "'I Wish' was gonna be the first song we were gonna do."
While Jigga has always had an R&B-friendly flow that gels well with a number of other artists, Tupac never played well with others. His solo singles rarely featured collaborations, and even then, he was usually alongside another rapper. That's not to say that he didn't dip his toes in the R&B waters ever once in a while (most notably, his work with Digital Underground always brushed up against those kinds of songs), but he was predominantly a harder-edged MC (albeit with a sensitive soul).
Tupac never really needed any help crossing over (he had Dr. Dre's assistance for that), but Kelly could have been the guy to help Tupac bridge the gap from his early work into the middle part of his career (had he not been tragically cut down in 1996, of course). "I Wish" is a pretty stellar song, and the addition of Pac's trademark voice (which was always full of both toughness and sadness) would have been a great start to a classic collaboration that never was.