Conditional product variant/attributes depending on other attributes. The variant/checkout attribute will be displayed/activated depending on selected value(s) of another variant/checkout
Example for selling printed t-shirts:
NAME ATTRIBUTE CONDITION TYPE VALUES
Printing Image printed in your t-shirt? none radio buttons Maddona|Elvis|your photo
Color Color of your t-shirt none radio buttons black|white|red
Photo Upload your photo Printing = your photo upload file
Name Your name printed? Printing = your image dropbox yes in red|yes in black|no
Text Enter your name Name = yes in red|yes in black text box
In the case of checkout attributes this can be added to New York's suggestion:https://www.nopcommerce.com/boards/t/18315/feature-request-conditional-checkout-attributes.aspx