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Yeah... you need to define your own Liscence in the EULA, from what I understand, as long as you write it in the EULA, it is enforcable by law.
My sugestion is that ALL mods, including base and core from the game, default to a liscence that says you can do whatever you want with them, as long as the original auther retains credit, and it remains for use with the factorio game product only.
Though, if you're going to do that, you might need to change a few images, since, to name but a few, Iron Plate, Petroleum Gas, and Sulfuric Acid use icons taken from elsewhere, Iron Plate from final Fantasy XV (I found the original image by accident while looking for images for metal plates), and the other 2 from Wikipedia. (Though the guy who made those chemical icons did so for public use, using a free to use program)