Start with what worked the first time.
And then make variants inside of it.
- Instead of a Pong Paddle, the player gets a character (a simple one)
- Instead of a Pong Ball, the game uses a "Razor Ball"
- Instead of trying to make a Pong ball pass a Pong paddle, the players will try to hit their opponent with the Razor Ball.
How do we make this interesting as a game?
1) Make about 5 to 10 different characters who each have their own bonuses inside of the game.
2) Make the levels(or arenas) interactive and with obsticles.
(Meaning that there are objects or elements inside of the arena that the players may throw the Razor Ball at, punch, or jump on to either receive some item or bonus feature [like "+5 life" for example], or trigger/cause an negative effect towards either their opponent or the arena in general.
3) For further customization, the players could be allowed to choose between styles of Razor Balls that they use as a character. Only one Razor Ball will be used during play, however the previously mentioned custom picked Razor Ball will be used on that characters home turf arena.
This is a very basic idea and I could expand upon this to just about infinate ends later on if you would like me to, but as the title suggested, you wished for a simple idea.
Just for a taste-test of what could be done.
- Players could create characters from a point-based generation system for the ultimate customization.
- Training for improved movements, and skills could be added.
- Team modes
- Alternate variations, like a skydiving Arena for Razor Ball.
Hope this helps