A part that allows you to select an image on your hard drive or the web and paste it onto the side of your rocket in a very similar fashion to the label part in terms of function and concept. Maybe you can not only customize the scale of the flag but you can stretch it vertically or horizontally to your heart's content. Additionally you could exploit this part to make textures for your crafts and such, like burn marks on a used booster or a shuttle. I think this would make many things much less of a pain and widen the horizons of building crafts in more detail.
(Oh and also if this does become a thing allow the use of images with a alpha channel like in .PNGs so some parts of the flag can be transparent. I think that this would be crucial to the part.)