To lengthen ... two thoughts
* what if you made each of the verticals one spiral longer?
or
* have more white space between the outside ring and the centre one, so that you are not equi-distance for both spaces between the 3 rings.
To quilt ... I see basic ... SITD to pop the cables/spirals, with a micro-stipple in the background.
Though I bet some will have some unique and much better ideas!
Should be fun for you to work on ... and I'll look forward to seeing it finished.