We could fix that. Two, or maybe three, serve at once, and your side loses a point if any hit the ground--or if you knock one out of bounds. Everybody has to be on their toes, including the scorer (did serve 1 hit the ground on your side before serve 3 went out of bounds?). The servers would, of course, try to overwhelm a position on the other side, so you'd need more players. And the strategy gets a bit more complex too--do you bat one in the air a bit longer so you can followup one ball that pulls the other team to one side with a ball to the other side?
Three balls might be too many, but..
Sometimes I write and then look things up. It turns out something like this is used a drill, except that each side serves at the same time instead of one side serving twice. Or thrice.