Good idea
This is quite a good starting point, the obvious things which need adding are simply making it longer and more dificult, which I'm sure will happen my default if you do make a full game from it
Secondly, Although this a purely asthetics, and for a game like this lines and sqaures work fine, but sometimes having better graphics can make the game seem more fun.
Thirdly, the idea for it dissapearing overtime is good, but maybe instead of it dissapearing as you move, maybe make it disspear over a certain time, or even have it so you complete a level with a visible object, and once you've done that you have to do it again, with the invisible object, but the idea now is good anyways.
Finnaly, if this does become a full game, I think a good way to improve the challenge of it would be first- Allow colour customisation of your object and secondly have different levels use different shapes, so levels where you guide a square and then one where you guide a circle etc.