I like a puree with almost anything well sauced. In this case, at this time of the year: Potatoes pureed with butter, cream and either a little parsnip or rutabaga. I think you were involved in the "use a ricer" thread. Anyway, for a dish this formal you're looking for something very smooth and light which hasn't lost any body. In other words, use a mill or a ricer -- not a food processor, masher or mixer.
A puree of white beans would be very good. That will get the Cuisinart humming.
Sauteed potatoes are never misunderstood.
A mix of wild and brown rices with nuts and herbs is also seasonal and a good pairing with duck. Considering how much is going on with the sauce, keep the herb choice simple. Just tarragon and marjoram would be fine. Sometimes I mix white rice into the mix -- but you have to cook it separately because it cooks so much faster than either brown or wild. I like to use chopped walnuts or pecans to enhance the wild and brown rices' nuttiness.
Another "no brainer" classic: Rice pilaf with almonds. Perhaps a saffron rice. In any case, use a fragrant rice like Thai or aged Indian basmati.
Wild rice on its own is overpowering.
Orzo. Orzo with rice. Orzo/rice pilaf.
Buttered (not too much) and herbed bow-tie pasta.
Chef petal suggested couscous. Her way, plain, or if you desire, served in a mound with chopped parsley, seasoned (but not soaked) with a mix of rice vinegar and lemon -- the pairing of the almost-tabouleh with the duck's sauce would create a sort of gastrique. Nice contrast, but maybe too Mediterranean? With my background it's a trap. With yours -- it's probably a strength. Bears thinking about.
Bulgur wheat -- either plain, buttered; or the warm, restrained tabouleh. I find bulgur wheat works extremely well any time you want something nutty. Speaking of which you could work in some pistachio in bulgur or couscous. Or, the rice for that matter.
Grits would be outstanding. A lot to choose from: Regular, butered grits; baked grits, cheesy grits; fried grits... I'd go with the regular, buttered sort -- consistency, a tiny bit on the stiff side -- like pureed potato