People,
I want to point out that in the beginning, both TOS and TNG had a core main cast, along with lots of guest stars as officers on the ship.
In TOS's first season especially, we saw several different helm and navigation officers, like Hanson, Riley, Spinelli, and Stiles, one security chief, Giotto, and any number of other science, command, and operations officers, like Boma, Gaetano, Jaeger, LaSalle, and the ubiquitous Leslie.
And in TNG's first season, there were several engineers, Argyle, Logan, and Lynch, a few conn and ops officers who rotated on and off the bridge, such as O'Brien, Solis, Su, and Torres, as well as a parade of other command, science, and operations officers, far more than in subsequent seasons.
Now, these were guest stars, not regulars, but still, you get the idea.
Come to think of it, NYPD Blue was like that in its first season as well, with a number of guest stars serving as detectives in the precinct before they settled on using the regulars all the time on all cases.
Red Ranger