I guess theres a few things you could do...
* Rotating paddles so the ball bounces off in wild directions.
* Have the ball speed up if the paddle is moving when hit, like a smash!
* Add obstacles onto a arena to deflect the ball.
* Allow the ball to spin, so if you are moving the paddle the ball bounces off spinning to suit - so it would be tricky to work out how the ball will bounce when it hits the a wall. Imagine hitting the side of the cueball when playing pool.
* Use 2 balls, so the goal might be to hit them at different angles to disrupt the other player.
* Have the paddle get smaller and smaller as the game goes on, getting even smaller when you loose a ball.
* Have a sorta bat on the paddle that you can swing round to hit the ball at full speed, like it would be a challenge to hit the ball like that - but at the end of the day Pong is all about skill.