As a tennis player myself, I feel that passing shots/volleys would definitely add the spice to this game, it would be super exciting.
If you could find a way to make it so that you would need to be well position and have nice timing/direction to volley. And also, if you volley to short/slow, it would easily be capitalized on, since you could easily hit the passing shot (as a pro player). Moreover, it would be exciting to introduce some way of skill passing shots (like a super low percentage shot).