Any baseball off-season weight training program worth its salt needs to include some type of progression. Although strength coaches and players tend to use progression when training most muscle groups, they sometimes neglect to apply it to core training. Throughout the pre-season, crunches and sit-ups are casually added to the end of every workout, with volume as the only measure of progress. While technically this strengthens the core, it's not the most functional way to train for baseball, or any rotational sport.
Start treating the core like any other muscle group. Begin with basic exercises and build up to more advanced exercises over the course of the off-season. Here's what a successful core progression series for baseball would look like:
Med ball throws are the best way to imitate baseball movements and generate speed. Rotational Med Ball Throws, Med Ball Slams and Shot-Put Med Ball Throws are a few of the many exercises you could do. Your imagination is the only limit here.