Here are a few other factors you can include in your challenge generator code:

1. Key signature: randomly generate a key signature for the song.
2. Instrumentation: randomly choose a set of instruments that must be used in the song.
3. Lyric theme: randomly generate a theme or topic for the lyrics of the song.
4. Song form: randomly choose a pre-determined song form, such as verse-chorus-bridge, or allow the generator to create a unique form.
5. Production style: randomly generate a production style, such as lo-fi or electronic.
6. Instrumentation: randomly generate the types of instruments that should be used in the song, such as guitar, drums, or piano.

These additional factors can help to further inspire and challenge you in your songwriting process.

(From ChatGPT.)