What can you do with Mixolydian mode?
It sounds like major but with lowered 7th scale degree.
I've been working on my Mass for the 2nd Sunday in Advent and today would like to share with you the video of the Prelude: Populus Sion.
As you can see, it only has white keys but the starting and ending notes are G's. Therefore it's a piece in Mixolydian mode. The Gregorian chant Introit "Populus Sion" sounds in the pedals. Enjoy!
PS If you feel the urge to try your hand at composing for the organ, know that it's never a bad thing to write down and develop your musical ideas.