I love your quilt! It looks warm and comforting. Personally, I would FMQ in the empty spaces, but you said you were not comfortable with doing that. That's ok. I like the idea of more echo quilting or the grid quilting in the blocks. I think that would make the cups really show up. As for in the cups, the heart would be nice or maybe a 'C'. It's a great quilt and it would be a shame to have the batting bunch or shred for lack of quilting. Good luck and show us the finished results.