I'd keep polishing it for a bit, but put the time into game feel rather than more features. Pong is a good project for that, since the game is so small that every bit of feedback you add shows right away.
These are the classic talks on it:
Juice it or lose it - Martin Jonasson & Petri Purho
youtube.com/watch
The art of screenshake - Jan Willem Nijman (Vlambeer)
youtube.com/watch
Can We Make This Button Fun To Press? - Jonas Tyroller
youtube.com/watch
There's a lot more on YouTube if you search "game feel" or "juice".
In Construct, two habits get you most of the way:
1. Animate with the Tween behavior instead of just setting a value.
2. Look at the triggers you already have (On collision, On tween finished, On timer...) and ask what the player should see or hear at that moment.
For example:
1.
Squash and stretch - set the size on a hit, then tween it back with Out Back so it overshoots a little.
2.
Screen shake - Camera is an invisible sprite in the middle of the layout with the Scroll To behavior. Your layout is the same size as the window, so turn on Unbounded scrolling in the layout properties, or there's no room for the view to move.
3.
Hit freeze - stop the game for a moment on a paddle hit. Untick "use time scale" on the Wait, or it waits forever.
4.Sparks - a Particles object with Type set to One-shot, created at the ball. It sprays once and destroys itself.
5. Sounds - one for the paddles, a softer one for the walls. This made the biggest difference for me. bfxr.net is good for quick retro sounds.
After a point - a sound, a bigger shake, and the ball fades in before the serve. That's also the pause before the serve you asked about, and R0J0's choose() can pick the side.
Last, once it feels good, try dressing it up. Swap the art for a table tennis table with two players, a tennis or badminton court, two goalkeepers on a football pitch, or anything else you like.
Keep the collisions and the art apart: leave the plain Ball and paddle sprites doing the bouncing, make them invisible, and add a separate sprite for each one that only shows the picture. Attach the picture to its sprite with a hierarchy (Add child in the Layout View, or the "Add child" action), so it follows along. Then you can swap images, add animations or make the art any size you want without touching how the ball bounces, and the squash and stretch can go on the picture instead of the real ball.
It's surprising how much a theme changes how the same game feels.