Instead of 2 values of transparency (1 or 0), I think it would be cool if there were 8 different levels of transparency. (any # between 0 and 256) / 32 = transparency level rounded to nearest integer.
So if a png file has pixels with an alpha value of 192, it will be level 6. But if it had a value of 150, it will be rounded to 5.
Reason why I suggest 8 instead of 256 is because for all i know, it might be computationally demanding.