Former player and current coach Jess Moore goes over several throwing plays for pitchers if they want to isolate a certain base as their focus.
If the pitcher's priority is at third base: If you have a runner you feel is weak getting to third base, it might be best to focus on keeping her from getting to third base. In order to keep good throwing vision, it's important for the pitcher to scoot several feet deeper into the field rather than scooting up closer to the catcher. This will allow the pitcher to keep both third and first base in her line of sight.
If the pitcher's priority is first base: The pitcher should practice throwing quickly and efficiently to first base, with little hesitation. Challenge your pitchers to make quick, strategic decisions about what base to target in every single play. This will depend on the strengths of the other team's hitters.