What should I add to butternut squash soup to bring up the flavor?

A dollop of creamy goat cheese :) Plus the Sherry vinegar, ginger, and sage as others said.

When something like that tastes a little flat, I find that the fix (besides maybe salt) is often a little acid - with your soup, a splash of either sherry vinegar or apple cider vinegar perhaps - not enough to make it taste vinegar-y, just a bit to brighten it up. Also as a garnish, a drizzle of toasted pumpkin seed oil or balsamic vinegar reduced to a syrup adds a little something.

Garam masala!

I like to use cumin, in addition to warming spices such as cinnamon and ginger. Freshly cracked pepper is also fantastic.

Try a drizzle of pumpkin seed oil

Ginger, Lime or coconut.

We like to add a splash of vinegar, extra ginger and fresh grated parmesan (not for on top but in the soup). Delicious.

The Silver Palate Cookbook has a wonderful curried butternut squash soup with curry powder, apples, and apple cider. It is delicious, and you can regulate the amount of curry you use. http://www.tastebook.com/recipes/1706756-Curried-Butternut-Squash-Soup I use apple cider in the place of apple juice.

How about Apple cider?

Sage! Goes with squash amazingly. But yeah ginger would be nice too.

I think a splash of sherry (or something similar) is a great addition to butternut squash soup. Ginger is also nice. Also, make sure you've salted it enough -- that will help to bring out the other flavors.

I like fresh ginger and some curry-like spices....and a good dose of freshly ground pepper.