Willamette is a distinctive hop, earthy and spicy. A hop like Mosaic might balance it well but you need to be aware that your beer might lose its Amber name and roll into an IPA instead. Mosaic has that resinous quality while just tipping into the circus side. IMO if you add something too far apart from Willamette the hops will become mushed together and lose all distinction. Cascade and Centennial always play well with each other as well.
Your Block Party has a single hop addition at 60 mins. You could also play with the 20min, 10 min, and Flame Out additions to make more hop flavor and aroma.
