I like a 100% cotton quilt i got from crate and barrel - it is 100% cotton filling and 100% cotton shell.

It's not a duvet, but i layer it with other things. Is warm and breathes well. Its a great mid-weight blanket.

I would also recommend looking at west elm. Or other similar stores. They are pricey but you can probably find what you want on sale or with a discount coupon.