The original plan was for Spock & McCoy.
If Nimoy had agreed it is most likely Kelley would have as well. In Nimoy's autobiography he said they discussed it on the phone and they both agreed that they had good ending in movie 6.
Nimoy also didn't like the role he was given in the movie. He was always very concerned with what Spock was doing and I say good for him. He obviously cared about his character.
I bet he would have done the cameo if he'd agreed to direct, but since the changes he wanted to make to the script were vetoed by the studio due to time constraints, that was never going to work.
It would be interesting to know what kind of changes he would have made. I'm willing to bet it would have been a much better film with his input.
He's right about the way "Spock" is written in Generations. Scotty and Chekov get some truly awful dialogue; Scotty especially is teching the tech in true late-TNG fashion. It's painful to watch. Imagine if they'd make Nimoy say that crap?