Star Trek: Generations only brought back three of the actors from the original series, but that wasn’t the intended plan
In the planning stages, Star Trek: Generations had a place for all of the original crew members of the original series. According to Ronald D. Moore, who, along with Brannon Braga, wrote the screenplay, the opening was longer than in the final draft and involved Kirk’s crew getting the new ship, Enterprise B, out of space dock. Kirk would have been in the captain’s chair and Spock at the science station as though no time had passed. Then a powerful energy field threatened the ship, and while the new crew of the Enterprise B tried to take control, but they were too inexperienced. That would have led Kirk and his crew to step back in and take over, returning to the positions they held aboard their Enterprise.
But the writers realized that they only had a limited amount of time to introduce the B crew and bring in the original actors, who were only going to be present for “maybe the first fifteen or twenty pages” of the script, according to Moore. That meant some of the actors would not have lengthy speaking roles, some even going pages without saying a word.
Moore and Braga knew they were not going to be able to make the scenes work so they rewrote the script with only William Shatner, Leonard Nimoy, and DeForest Kelly in the film. After Nimoy and Kelly turned down the chance to appear, James Doohan and Walter Koening found themselves back on board the Enterprise with Shatner.
It’s unfortunate that the original script didn’t work out. It would have been a fitting final sendoff if the entire crew had been present when Kirk fell into the Nexxus. They would have all had the opportunity to say goodbye to the captain they’d followed for over twenty-five years Plus, fans would have gotten the chance to see the crew in action one more time.