@Angle Frogatto came up with a solution for this, which some other games do. Basically you can license creative and art assets past some "demo threshold" (end of 1st level, etc) under a different license, or just reserve all rights for them. That way people who want to modify the game can do so, but they can't just republish a barely changed entire retail release, since they need to make the art for the levels past a certain point.

So in this setup, all the code is under whatever open license you prefer, and all the art and sound for everything in the "demo" section is under an open license as well. However, past that point all the art and music (and assets like level data files, etc) is a seperate data-pack from the full retail version which can only come bundled with your own official retail release (since you retain all rights, and the assets can even be free of cost if you want). Since this requires splitting up your game into two legally distinct parts, you can distribute them in two packages via package managers too (you have to install "game" and "game-data" for example).