This last stage really sets the stage for specific soups
At this point some decisions will need to be made, what flavors are you wanting your soup to focus on. Adding tomato at this point can be done in 2 ways. If you have fresh chopped tomatoes, toss them in now. This will give you a smokier tomato flavor, if you want it a bit milder add some liquid then the tomato.
Green pepper can be added as well, this is to impart a touch of bitterness, done before adding liquid
Leeks can also be sweated in at this point.
Basically add any flavor that will survive the sweating in process. Sure you could add sweet peppers, but really there flavor will actually be diminished by the time your soup is done, better to add it towards the end. Same with sweet potato or yams, they cook so quickly and give up there flavors so fast that a long cooking time is not beneficial, leave it towards the last 25-30 minutes of your soup cooking.
The addition of certain flavors like the tomato or green peppers lend them selves to soups like minestrone, heavier flavored bean, pea and lentil soups in addition to kale soups like Ribollita or Zuppa di Verdure all' Agliata an intense garlicky vegetable soup.