Looking really cool so far! I think you could even take it a little bit further with the realistic slinky dog. You can buy, or even have made, slinky coil spring wire in just about any dimension you want. I was able to find Slinkys out there that are in a few different sizes from 1.5" to 2.75" in diameter. You could get one of those, replace the rings you've got there already with a genuine slinky spring and have a couple small, clear plastic tabs mounted to it in a couple spots so that the ball lightly taps them on the way down the wireform. They shouldn't be so stiff that they slow down the ball all that much, but enough to animate the slinky in a really cool way. The head tilting would also send a nice wave down the slinky, too. You'd most likely have to weld the slinky on at the front and back of the wireform, but you could most likely use two extra wire rails to lock it into place and keep it upright, but in a manner that allows it to move freely.
Either that, or you use the mounting points of the dog head and turn it into a pivot point where the head moves forward a little bit when the ball hits it and that is what would animate the slinky.
It's such a neat idea, you've got MY creative juices flowing! No matter what have fun building!