Oh D, That is so pretty..please tell me you are doing that freehand?? If so ..quit your day job! For sure! I keep telling you!
Anyway...as far as the triangle...is ruler work an option...if so I would do graduating triangles.....or you could do the wavy outline and keep going round and round until the area is filled.
You could put a fan in there.....a flower that fills the space...SO many options.
I love it!
You and I love the same fabrics!