I think there was some consideration given early in the process to recasting the characters with major actors, like maybe Robert Redford as Kirk. So maybe that's what would've happened, at least for the lead. Then again, with Hunter in the lead, given his lack of enthusiasm about television, the show probably wouldn't have been as popular and there might not have been any movies. Then again... Spock was the runaway star all along, and Shatner was fighting to stay central, with Roddenberry struggling to find a way to balance them. Hunter in the same context might've been more willing to let Nimoy take the lead, and we might've ended up with Spock becoming captain in the second season (and since this was '60s TV, there probably wouldn't have been more than a line or two of dialogue saying that Pike had been promoted or transferred or something).