I'm in my church right now and the first thing I did when I came here very early in the morning was to improvise this piece "Populus Sion". It's based on the Gregorian chant Introit melody for the 2nd week in Advent.
The tune is in the 7th mode which starts and ends on the note G. I used deep foundation registration for it - on the 1st manual - principals 16', 8' and 4', flutes 8' and 4' and on the pedals - principals 16' and 8' as well as flutes 16', 8' and 4'.
On the psalm verse I jumped to the 3rd manual and played with flutes 8' and 4', gamba and principal 8'. Then I returned to the 1st manual. but increased the initial registration gradually up to the mixture in the manual and posaune 16' in the pedals.
Generally speaking, I played almost without any sharps and flats, just the diatonic notes. But towards the end, I added some open fifth sounds and chords of Eb major and Db major to make it sound more colorful.
Let me know what you think and now I will go meet @laputis for lunch after which we come back to the church and hopefully practice our Christmas organ duet program for the first time.