What are your favorite ways to fix a boring soup?

A little worcestershire sauce can help. Also, pureeing part of the soup can make it thicker and more appealing.

A splash of wine, sherry or vermouth. Where is Julia when we need her? A good glug of wine will make a boring soup good.

Tabasco

... another thought. You can make dumplings from cream of wheat (as in this recipe https://food52.com/recipes/20588-choucroute-soup-with-knoedels). They are very simple to make and taste like snowflakes. They are a popular addition to soups in Central and Eastern Europe. You can also add some parsley to the "batter", or other herbs and spices.

If it can handle being cooked a little longer, throw in some parmesan rinds!

Croutons fried with garlic and olive oil. I have to do this from time to time and it really improves the soup.

In Chinese cuisine there is a wonderful broth flavored with star anise, cinnamon, ginger, scallions, and soy sauce. There are many variations on the web... Here is one I like to make http://www.foodandwine.com/recipes/chinese-poached-chicken-breasts-with-star-anise. I also make a Westernized version of it, see the recipe for Christmas Duck Soup with Star Anise here, http://www.threelittlehalves.com/2014/12/poor-mans-christmas-rich-mans-christmas.html. There is also a nice recipe in one of the Fuschia Dunlop's book, but do not remember which one -- need to look it up.

I made a puréed carrot soup the other day that seemed kind of lacking. I added some fresh grated ginger root, some grated lemons rind, and a bit more salt. That really brightened the flavor. Then for more body, I did something I've never done before -- I added some leftover hummus. It's pretty darn tasty now!

You could treat it like dal and add a spiced oil at the end. Sautée garlic or ginger and some spices in a tablespoon of hot olive oil until they are fragrant, then add to the soup and let it simmer to incorporate.

Mushrooms can bring umami to veg stock, which adds depth. Also, I would ask these questions:

Is there enough salt?
Is the broth reduced enough?
Are the sweet, bitter, and acidic components balanced?
Is there a fatty/starchy element (like an avocado or cheese garnish or some pasta or rice)? I think a little fat and/or starch can help out a veg soup a lot.
Could it use some heat?

That's probably where I would start with trying to fix a boring soup. Let us know what you wind up doing!

I second lemon juice/zest or even thinly sliced preserved lemon. A thick balsamic drizzle it also helpful. A really good olive oil always goes on my veg soups at the end too.

homemade croutons on top.

all or some:
tomato paste
canned beans plain or flavored
Zartarian's packaged red beans and rice or black beans and rice
okra, cut into 1" lengths- frozen or fresh
persian dried limes
chopped dill, parsley, cilantro- lots of
parmesan
salsa
lemon juice
pesto and pasta
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for asian slant: minced sauteed ginger and coconut milk, lime juice.

You could add some cheese tortellini or mini ravioli.

Adding some kimchi can help some soups.

Chopped parsley (lots of it), plus a generous spoonful of a sharp stoneground mustard (I like one with horseradish in it!) Vinegar or lemon juice will also brighten the soup up. And are you sure it has enough salt and freshly ground pepper? ;o)

pesto!

I love a scoop of sour cream in veggie soups!

Certain soups benefit by adding a little vinegar at the very end.