Hitting a baseball is often considered the toughest thing in sports. The ball arrives in less than half a second and in that time you have to calculate the trajectory and determine if you want to swing at it or not. Also the ball is hit with a round bat which is unique to batting sports, a round object really isn’t ideal for hitting.
Here are some requirements. If you identify them correctly you can train them separately with drills like classes in a school curriculum or integrated in a holistic approach. You can also create a scoring system.
In order to get around to 90+ MPH and hit the ball hard you need batspeed. The harder you hit a ball the batter the results generally and this is mostly dependend on batspeed. So building batspeed is very important as most good MLB hitters are able to swing at 80+ MPH.
You can increase batspeed for example with an overload underload program as it is found at driveline baseball https://www.drivelinebaseball.com/hitting/ and also this side
2. A balanced turn: Baseball hitters turn in their swing to generate and store energy. To support a good bat path you need to turn while maintaining good posture (lateral and anterior tilt, a still head. From the side view the head should stay centered or even over the rear hip. It can help to pull the rear thigh under the body during the turn and skip the back foot forward but not everyone does that. Baseball rebellion has some good drills to teach that. https://baseballrebellion.com/cpippitt/the-movements-that-made-the-rebellion/
3. The ability to swing on a good plane: Ideally the bat should travel upwards 5-15 degrees at contact. This is to match the plane of the incoming pitch and to generate loft in the swing which creates power. Posture can help with this as well as a good hand and bat path. Things that can help specifically is lateral tilt to the catcher, the front elbow and shoulder working up, the bat being turned down behind the ball early.
4. A good direction through the ball: An old saying was short too and long through the ball. The swing is an arc around the body but it helps to elongate the arc to an elipse so the bat stays in the zone longer and doesn’t roll over tot he other shoulder too early. This also includes good extension through the ball which can occur before or after contact based on a lot of factors.
5. Deceleration/Anti-rotation ability: According tot he forthenbough paper „the biomechanics of the baseball swing“ the maximum angular velocity of the body is achieved about 60 ms before contact and then it slows down to transfer energy. Rotation is important but so is bracing and stability which helps with both balance and direction through the ball as well as batspeed. Also helps on outside pitches on which the body decelerates earlier to allow for a better direction to the other field.
6. A good loading pattern: muscles must stretch to fire at maximum efficiency. Both the lower body (coil) and upper body (hand cock, back elbow row, scap load whatever) must load. Ideally the lower body loads before the upper body so that it can also unload in that sequence.
7. A good unloading sequence: sequence must start in the lower body and continue upward into the trunk, hands and bat.
8. Good hand movements: A lot of people talk about turning the barrel to get the bat down on plane early. I think this thing is overblown but still having effective hand articulations (unlar devation, top hand supination, not too early lead arm pronation) can help delivering the barrel behind the ball more efficient and on a better path.
9. Ability to adjust for timing: Pre pitch: start loading sequence later. Pre toe touch: try to slow down forward movement if you recognize breaking ball mid air. Front leg absorption: this will hurt ideal batspeed but you can sink into the front leg and delay the upper body. Using extension to catch ball out front is the last string. To help for this it can help to hit many balls with bent arms a little deeper to have room for extension but not too deep either as optimum batspeed occurs around at the front toe. Train to hit different speeds.
10. Ability to manipulate the barrel path to all 4 corners oft he zone. Get behind the ball in every zone. Practice hitting balls in all locations.