What if Leonard Nimoy had directed "Generations"? The lowest moments of GEN were all parts with original crew. Chekov and Scotty we're OK, but their plot lines were strange. Kirk's death... Well, it was unnecessary (IMO). Maybe Nimoy could improve something or even remove scenes with original crew.
In my opinion, Nimoy would have been a step down from Carson. The direction was not the problem with the film, and Carson acquitted himself very well behind the camera. (It also helped that John Alonzo's cinematography was top-notch.) Nimoy's direction, in my opinion, was never lively.
What if David Fincher had directed Star Trek: First Contact?