I've read that Jack Lord was offered the role of Captain Kirk prior to William Shatner getting the role. Captain Kirk was created to replace the character of Captain Pike from the first pilot episode after Jeffrey Hunter balked at reprising the lead in the actual TV series. What if Lord had been offered the earlier role of Pike instead of Hunter? Would Captain Kirk even exist?
Similar with Mr. Spock. Apparently Martin Landau was offered the role.
What a change these two actors would have made!