Haha! I have been pondering this as I always wear down coats in winter. Last year I wore mine with pretty much anything and didn't care, but mostly a leather shopper or cross body. Depends on the colour too: my navy coat went well with a leather Jaeger tote in reddish-brown.
I think a lovely leather satchel bag would be most fitting to the style of down coats of any colour, probably worn cross body.