Depends on what I am cooking. I don't usually batter fry my ducks because they have a thick layer of fat on them already (deep fried fat? Hmmm....) But if I did, I would use a spicy mixture (chipotle pepper or hot paprika or mustard powder with salt, pepper and garlic powder. Here's the catch: Dust your duck breasts with this BEFORE you batter them. Spices and hot fat hate each other - the spices burn and turn nasty. But when they are under a coating of batter, you get the flavor without the burnt taste.
As for a marinade, if you are looking to tenderize something, submerge it in buttermilk a day. If you are looking for flavor remember that any good marinade has four key ingredients: Something acidic, Something sweet, something salty and something oily. Remember this and you can do anything: So maybe lemon juice, olive oil, honey and salt. Or soy sauce, red vinegar, brown sugar and sesame oil.
Hope this helps.
I love all of God's creatures, properly prepared.