For the love of all that is holy, please don't create new licenses. There is no need.
Simply license different parts of your project under different existing, well understood, well documented license.
e.g. code under GPL or MIT, so people can share it and re-share their changes, and assets such as graphics under a more restrictive license.
This has a common practice for many, many years, and will allow exactly what you want to accomplish. People can redistribute code or games based on things they've learned from you, but will need to provide their own graphical assets and music.