So many nice ideas being posted on this Topic. Thank you Scottyee for starting it.

Like most, my approach is basically to use a Fill (and change in variation) to mark a transition to a new section of a song.
On the Ketron X1, however I sometimes introduce a change by activating Intro 1 instead of a Fill. Intros 2 an 3, may not be suitable because they can be too fancy and may sound unrelated to the music theme, if activated in the middle of the song.

Another technique is to use the KeyStop followed by pressing a Fill. On the X1, with the KeyStop on, the rythm stops if the left hand quickly strikes the chord and is released immediately. After the music stops you can just press one of the Fill buttons and then, when you resume action with the left hand the music starts again with the selected Fill.

Finally, my old Technics KN1000 has a button for so called "Dynamic Accompaniment". (When the button is active), the idea is that depending on some key velocity/chord/right hand combination, the instruments introduces a random fill or variation. To be honest, I have not been able to determine exactly what combination triggers the event, but it works and would be nice to have something similar on the X1.