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.
But I do think Kelley would have done it if Nimoy had agreed. Although I also read that Kelley was ill at the time. But I haven't seen anything official to confirm that.
I have not seen the movie and I don't think I really care to see Kirk's death but if Spock & McCoy had been it I would have been more interested, I love the trio.
I do like Scotty & Chekov too of course but it just isn't the same for me.