You keep the girls who want to return. They've been foundational for you, and it's just kind of crappy to tun them loose. You let everyone know you're expand your roster another couple of spots, and you find the girls you like.
Then you have team tryouts on a very regular basis. You give girls the chance to compete for positions. Make it as empirical as you can -- you hit this many grounders to girl A, this many to girl B. Whoever fields the most clearly gets the start that week. Same thing with pop flies for OFs -- what err criteria you want to use, use it and give any girl who wants to compete for that spot the chance. The girls who don't win will have a reason why, and they can decide to try harder or they can understand. You're not creating a culture of shoving people out when they aren't useful to you anymore, and everyone can feel as though they have a real chance to play.
One of the things that has been missing on a lot of teams I've been around is accountability. A coach designates someone a shortstop, or an outfielder and other players don't get a real chance to compete for the position. The coach's eye may be right, but it's better for everyone if the anointed one has to prove it week in and week out. Plus, because you've got others vying for the position, you've got someone who's been working at it when you need to sub. And if the two girls you're questionable about are given the chance/shown they have to compete for their positions, they may take things a lot more seriously next year.
How about the girl who doesn't come to practice and gets the start anyway? Or the girls who miss the first game and a half to take the ACT and immediately get put in when they arrive midway through a game? Why is accountability only discussed in terms of the player's responsibility? Does the coach have no accountability in terms of playing the girls he CHOSE for his team? I'm not directing that at you in particular, just trying to illustrate another viewpoint.