Re: What if Gary Mitchell didn't die?
He would have played the roles of Bailey in "The Corbomite Maneuver" and of Lt. Styles in "Balance of Terror". Possibly someone with an attitude like "hit them hard, while we still can" or "fire first and ask questions later".
Come to think of it it might have been pretty interesting.
"The first duty of every Starfleet officer is to the truth, whether it's scientific truth or historical truth or personal truth! It is the guiding principle on which Starfleet is based! Jean-Luc Picard