Jan, a 15" block is a lot of space to deal with at one time, especially on a domestic machine. If I were doing that kind of project, I would divide the blocks into quarters so that there is less area to deal with at one time. I would probably use a walking foot to divide the entire quilt and stabilize the sandwich then quilt something in each smaller triangle like Kathy suggested.
Do this stitching (the entire quilt) with a walking foot:
Attachment {!-- ia1 -->hst for jan.JPG{!-- ia1 --> not found
and here are just four ideas for the smaller triangle spaces. There are LOTS of motifs that will work there! Have fun, and please show us a picture when you get it done!
{!-- ia0 -->jan.jpg{!-- ia0 -->