Beautiful bags, everyone!
Remember that just about anything that can be made into long strips can be crocheted into a bag. Worsted-weight yard does produce a rather thick bag, and cotton thread takes some time,,,there is also sport yarn (what some refer to as baby yarn) and it comes in nice colors, many with threads of gold or silver woven in.
Also remember that cloth can be used for crocheting as well. Cottons are especially easy to work with. It's like when you use blue jeans to crochet a rag rug; just cut the material into thin strips, and you're off...you can knot the ends together to make a rustic type bag, or simply work them in as you go.
I'd love to see the ribbon bags, they do sound lovely.
Also, the flexibility and stretch of your bag is going to basically be determined by the stitch you use, and how tight your stitches are.