yes! when I was young, I did the same thing, maybe not with a drawing on the window, but sometime just with a finger print or maybe a toy or anything in this kind, when I presented the game on the local, I figured out that I wasn't the only one who was playing this young, as soon as I begin to explain my game some person begin to laugh and told me that they was playin this young too : D
no its not, and there is and ending, but in case of an endless I would have added a Score systeme first, and maybe add speed as you go in to the game to increas the difficulty, that would be the easyest way, the second way it would have been a spawner of the graffity, wich spawn graffity in a random range each X time, and then shrink that X, and add a systeme that prevent impossible patterns.
Thanks a lot Raed Tabani for your message :)