What if Gene Roddenberry wrote Star Trek when he was 26 and it got produced as a series of films in 1948?
Initial thoughts
Captain Kirk - John Wayne
Spock - Gregory Peck
Bones - Jimmy Stewart
Scotty - Cary Grant
Sulu - Yoshio Tsuchiya
Uhura - Dorothy Dandridge
Chekov - Anthony Perkins
Nurse Chapel - Marilyn Monroe
This means, at the least, that Peck doesn't get a chance to do Gentleman's Agreement and Twelve O'Clock High (nominated for the Oscar in the 1st, won for the 2nd). Depending on pre-production timing it might prevent Stewart from his defining role in It's a Wonderful Life.
Monroe was under contract to Columbia at the time. Wayne was contracted to Republic & Peck was working for Fox. The studio system was still all powerful in those days. It's hard to see how Peck and Wayne would be allowed to work in the same film by their respective studios. Monroe was a no entity at the time. There were literally pools of starlets under contract at every studio, whichever studio made the picture would have simply dipped into their retained talent pool.
BTW - I assume that this would be a movie? or a serial of shorts? TV shows were pretty much non-existent at the time.