I do not want to allow closed-source forks of the game, though I do
want to allow closed-source mods.

The license's effect on mods was not something I initially considered.
Only using GPL-compatible assets is a pain, one I've had to deal with
making this, I wouldn't want to force it on others if I can avoid it.
I'm also fine with people keeping their original assets closed.

Mods that include modified code or assets from the base game will
still need to be open source. This doesn't include entirely replacing
asset files with completely new ones.

(The above statements are not legal advice. See the license terms for
full details.)