I like boring accompaniments.. that way they don't compete with the stronger flavored dishes.
I'd stick to potatoes, and maybe roast them with other winter available root vegetables like carrots... or maybe fennel or parsnips. I don't like either of those so much.. so, I'd stick to potatoes and carrots... maybe throw in some some thin haricot vert type green beans. To roast the vegetables I'd stick to pepper, olive oil, and a good quality coarse salt. That way the duck can be rich, you could serve an overly rich dessert.. but the side wouldn't be so over the top that it would balance out the rest of the meal. A really good bread would be good also. I tend to like peasant food style cooking, so I usually cook with less ingredients than more.