It's fun to draw different shapes and see them flying everywhere. I think it should restrict us to only draw shapes in each color once and show the time of survival as the score.
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I do like the freedom of drawing as much as you like, though I have also been thinking of having a limited "ink pool" which would need refilling somehow before you can draw more. Your idea certainly fits in the same category, and it could be an interesting (maybe optional) challenge.
Thanks for playing!