I think that more yellow would be a lot of yellow, and could detract from the central motif. I'd use a thin cream and wide blue to echo the colors in the center.
And as for quilting, I'd change thread colors rather than use invisible.
I'd stitch in the ditch all around the motifs, SID the elf's face, and then do filler on everything except his face and hands.
I might use a yellow thread on his shirt and quilt straight horizontal lines to make it striped.
Is he holding a tree? Or 3 trees? I can't really tell.
I'd fill the background with ribbons.