There are two schools of thought with regards to gym stuff - try to directly replicate the swing but with added weight. Or do exercises that enhance overall fitness but are not directly related to the swing..
I feel for the most part option 2 is better - albeit something like the FYB "box" exercises (but with medicine balls) are okay. I don't consider them plyometrics though.
Chuck Kriese Power Tennis: (uses medicine ball)
The problem with the heavy racquets or adding weight to your swing and mimicing is that the evidence I have seen indicates this will slow your swing down as you are conditioning your body to moving a heavy weight slower - rather then moving a light weight fast. Both golf and baseball have done some research with this kind of thing..
So overall fitness is probably the way to go. I like exercises like planks, squats, lunges and more for that.. As well as some plyometrics like box jumps.