Vegetable stock or broth can be made with nearly any combination of vegetables. Just keep in mind that strong-flavored vegetables, such as cabbage, eggplant, turnips or peppers can dominate the flavor, so these should typically be omitted.
Ingredients such as corn, or even corn cobs, add a buttery mouth-feel to the stock (basically what gelatin offers to meat stocks). Celery root and parsnips offer some bite; tomatoes and dried mushrooms (i.e., chanterelle, porcini, or shiitake) can offer great depth of flavor.
A wide variety of herbs and spices, such as tarragon or fennel seeds, can add more complex notes. Even experimenting with items such as sea weeds, dulse, or certain legumes such as chick peas and lentils can give surprisingly satisfying results.
Vegetables stocks, due to the lack of animal protein and fat, tend to last a bit longer in the refrigerator. An excellent vegetable stock can easily substitute chicken stock in almost all dishes, so don’t think of them as second rate at all.