The films he made afterward included Brainstorm and Dimension 5, which had sci-fi elements, and A Witch Without a Broom, which was straight up fantasy.
Or Forbidden Planet.
As you point out, very difficult to know.
But you can take the first hand full of episodes produced (not aired of course, but produced) and compare those with the rest of most shows that run for a significant period of time and they change. Isn’t that always discussed with TNG? So it’s hard to know. But Hunter was a seasoned professional. I’ll presume that he would have developed as had the rest of the cast.
Jeff Hunter’s captain would have been a little more serious. His character reminded me a little of Picard in TNG. I think that’s the direction Jeff would have taken the character. I would have enjoyed watching Jeff develop this role.
Kirk was Captain Hornblower, a swashbuckling, romancing, action character.
The pilot was rejected for being too cerebral. Roddenberry made several character adjustments. The captain became larger than life. I lost track of how many women he kissed. 
Leonard Nimoy has said how Shatner’s broader acting style allowed Spock to come into his own. The subtle, often gently mocking Spock we know from TOS wouldn’t have existed without Shatner’s Kirk to play against. And it’s hard to imagine Star Trek being a hit with the weird, intense Spock we see in the pilot.