There are so many possible additions to this game that it's hard to even find a starting place. Here are just a few things that can greatly improve it:
Resolve the edge-effect issue. That is, of course, easier said than done.
Develop a frame-independent rotating line-ball collision-detection routine for the flippers. This is another thing easier said than done, but it is possible and not as difficult as it might seem.
Add in the "order of events" aspect of the typical game of pinball that we did not attempt here. For a complete game you would definitely need to add that feature. (See "The Skinny on Pinball" early in this chapter for a discussion of the general procedures and characteristics of a full game of pinball, including the order of events.)
Add in other typical pinball gadgets such as sunken holes, two sets of flippers, and tiles that can be knocked over.
Tilt the game to a slight 3D perspective to give it the illusion of being more real. Most of the better computer pinball games are rendered that way.
Add in a ramp to a second level. The ball can travel up the ramp and then score points by hitting objects on the second level.
Find or make some crazy sounds. Just about anything goes in pinball, so you should not have much trouble finding some great sounds to add to this game.
Add lots of blinking lights and flashy animations to the table. In a game of pinball, the table is almost never static. There is always something blinking.
Add the ability to have more than one ball on the table at a time. This is another addition that would not be that difficult.