This doesn't answer your question directly, but spices are only a small part of the picture. Below are some techniques to get more flavor in your soup.
Longer Cooking
Depending on the type of soup you're making, you may just need more time. Some flavors just need more time to get out. This is especially true of meat and bones. It's possible to make a ham and pea soup in an hour, but simmer that ham for 3 hours and it's night and day.
Rest overnight
Many stews, chili, etc. seem taste "better" the next day. Give an overnight rest in the fridge a shot and see if the flavor improves.
Add volatile flavors later
There is a caveat to go along with longer cooking. Some items work over long cooking times, while others lose flavor. In addition to meat/bones, garlic, onion, bay leaves (and others) benefit. Many fresh herbs, pepper, vanilla, saffron (and others) lose their flavor from the heat. If you are cooking for a while, you might try adding some more volatile flavors later in the process.
Add salt early
Salt helps to extract flavors, so add it early on in the process. Do be judicious with it, however. Long cooking tends to concentrate flavors, so you can easily make it too salty.
Sweat vegetables / brown meat
Before getting any water involved, sweat your onions, celery, peppers, mushrooms, or whatever vegetables or playing in this game.
Likewise, brown your meat. The maillard reaction is necessary for developing great flavor from the meat.
Typically you should use yellow onions for cooking. They have a higher sulphur content and are more flavorful after cooking. Raw, a red onion will taste more pungent. However, once cooked it is more mild and sweet than a soup would require. Red, as well as other sweeter onions, have more sugar and water than their yellow counterparts. For more details on the profile, seriouseats has good primur.
If you wish to improve your results, you might slice more thinly and add some salt in advance of sweating (rather than before adding broth) to draw out the excess moisture, as the onion's moisture (more prevalent with red onions) will inhibit browning, ergo inhibiting caramelization and the maillard reaction that break the bigger sugars into little ones and lend flavor through browning. The explanation at norecipes also adds that deglazing with sherry can slow down the process if you are experiencing uneven browning. Another possible tactic would be adding a 1/4 teaspoon of baking soda per pound chopped onion; this will speed up browning.
Ultimately, unless the recipe calls for red onions, you are going to get better results with yellow onions in terms of raw, "sweet and tangy" onion-y flavor. You can use the tips above to try to overcome that, but a nice big bag of yellow onions are going to do you better in the long run, and if you put the same effort into the it will take less time, taste better and be less expensive.
Best Answer
If too much water is the culprit, then its a simple matter of reducing the soup on the stove top. Just put it in a pot and boil it with the lid off(so the water escapes) until you reached a desired thickness and flavor.