William Shatner very nearly appeared in the prequel series Star Trek: Enterprise, but the plan fell through when Shatner and the producers couldn't agree on a creative direction. After attempting to set itself apart from most other Star Trek series early on — the words "Star Trek" were not even in the title of the series for the first couple seasons — Enterprise began telling stories aimed more at longtime Star Trek fans who had stuck with the franchise through thick and thin.
The show's fourth and final season would revisit some longtime Star Trek stalwarts, such as an ancestor of Noonien Soong, the creator of Star Trek: The Next Generation's fan favorite android Data. The series also explained one of Star Trek's great mysteries: why did some Klingons have forehead ridges while others appeared mostly human? The answer was complicated and convoluted, meaning it was just about perfect for Star Trek.
But chief among the fan-favorite ideas was the chance to bring William Shatner back into the franchise. Shatner's Kirk had a pretty definitive end in the feature film Star Trek: Generations, where he died helping Captain Jean-Luc Picard save a planet from a mad man named Tolian Soran. But Shatner and collaborators Judith and Garfield Reeves-Stevens had come up with an idea to revisit the Mirror Universe version of Kirk from the classic Star Trek: The Original Series episode "Mirror, Mirror." The time-displaced Mirror Kirk would have actually ended up creating the Mirror Universe with the unintended assistance of Captain Archer.
Enterprise showrunner Manny Coto was impressed with Shatner's pitch, but executive producer Rick Berman wanted to bring in Shatner in a different way. Berman had the idea that Shatner could play the Enterprise's chef. A temporal agent would have recognized the fact that the chef bared a striking resemblance to Captain Kirk, and would have brought the chef forward in time to pose as Kirk in an important historical moment. It's a decidedly sillier concept than the one Shatner and his collaborators came up with, and one that did not please Shatner at all. Enterprise was ultimately canceled before the producers could solidify an idea that made everyone happy.
The chances of Shatner returning to Star Trek again seem slim — the 89-year-old actor himself has suggested he's largely left the role behind him. Given that nobody was particularly thrilled with Kirk's demise in Generations, it seems like a genuine missed opportunity that Shatner couldn't get back in the saddle one more time on Star Trek: Enterprise.