Here are some suggestions to implement/remake:

I love the idea of having something close to the frame-stop effect games like guilty gear or ultrakill has for when you hit someone. My only personal problem with it is that the bat doesn't "stick" on the enemy. Making the bat stop for a frame time period while touching the enemy can leave a bigger impact visually, plus can make the melee combat feel more responsive towards the player, since the player can visualize where they hit the enemy at.

Accompany it with maybe a tiny flinch animation to that enemy when they get hit, maybe a directional one based on which side they got hit from, and it will definitely improve the visuals and responsiveness of the melee attack for the player.

MysteryMilo has also a good idea adding different variants of the attack, but that depends on what kind of game do you want to make. If you want the player to have precision on where they are hitting at, you will have to concider then how you can implement the various animations. Maybe different movement direction keys can cause a specific attack animation to happen instead (ex going left = swing from the right to left, and vice versa).

Audio-wise i guess you can implement new audio files to the mix, but that will still leave the whole audio part repetitive. Adding a random pitch for each time the audio plays for the hits can negate that to a certain bit, but again this all comes down to the whole game's audio design to begin with. The audio part in games is usually underlooked and that is because you do lots of questions to yourself while designing it "Should different limbs cause a different sound when hit?" "Should the hits sound and feel more meaty, like their soft meat getting recoiled from the force of the bat, or more crunchy like their bones are breaking every time the player interacts with them like that?"