Star Trek as a series usually sticks to its core cast. Throughout most of the shows and their seasons, we get the same handful of characters for the majority of the show. We'll get different recurring characters, sure, but for the most part, the main cast is who the main cast is. With a few exceptions.
When there are changes, they are almost always Starfleet officers from other parts of the Federation who come in as relief. Very rarely did we ever get to see non-Federation crew members be brought in as emergency members of the ship, and for good reason. It just wasn't needed. Why would you need to when another ship, space station, or base was always so close?
Yet, Star Trek: Voyager's entire premise revolved around this idea. Non-Starfleet members came together with Starfleet members to build a unified ship. Some will note that one of the two major factions who made up Voyager's crew, the Maquis, wasn't used well enough after their integration, but I don't think that was the biggest fumble the show made.
Early in the series, the Federation and Maquis were joined by a single Talaxian and a single Ocumpan; Neelix and Kes. Neither were Starfleet officers but both became needed members of the crew. Later, Seven of Nine, a former Borg drone was also added. All three remained unofficial members of Starfleet until they all left the ship. We'd only see this idea get used one more time when Voyager brought the surviving members of the other Federation ship stuck in the Delta Quadrant, the Equinox.
After the two-parter that introduced the Equinox ended, we would never see those surviving crew members again, despite being on Voyager.
Adding non-members of Starfleet to bolster the ranks was a great idea that frankly, should've been further explored. The concept that Voyager was so hard up for crew members that they would bring on other species and other ship's crews just to strengthen their ranks from time to time would've been a lot of fun to explore on a regular basis.
Yet, despite this, it wasn't really a concept we'd see much of and that's a darn shame. Since Voyager was in the Delta Quadrant, with no help and few allies, the concept of them recruiting aliens for their ship should've been explored more consistently. After all, Voyager was in a situation no other Starfleet ship was ever in.
With the ship constantly besieged by violent attacks and losing numerous crew members throughout their journey, you'd have thought that the Voyager would at some point start looking for new recruits. These could've added some interesting story elements that would have punched up some of the earlier seasons.
Not only that but with the ship traveling away from various points of interest throughout the series, recurring characters became harder and harder to create. Specific alien races were left behind and nearly forgotten the further the ship got.
Adding some new characters through recruitment scenarios would've allowed Voyager to have a few more recurring characters to pull from on occasion to help liven up otherwise by the number of stories.
Sadly it was something that wasn't really looked into outside of the couple of times that Voyager did it.