I finally decided on a verse/chorus idea and then set up a complete song form - a pretty standard one with a solo section over one verse/chorus. The turn-around at the solo and the last chorus is extended with a little build-up. After I put together the song form and melody for this song, I wrote some lyrics.
Then I put together this home studio demo. I made the piano part simpler and less frantic by adjusting the right hand arpeggio so it's more of a single note pattern up and down. And also the left hand arpeggio is only played every 2 bars. More relaxed. I added some drums, bass and a little acoustic guitar as I thought they should sound. This is the recording that I gave to the musicians as a guide for learning the song.