I just add whatever vegetables I have on hand. The corn tends to sweeten up the soup quite a bit, so I add a lot of that.
Not sure why your soup taste like dishwater. Is it possible the water you are using is contaminated with soap or detergent?
I don't have any particular recommendations as to what to use in your soup. Only that you use the best quality, in-season material you can get your hands on and also the water and hydrogen peroxide soak prior to cooking.
You can also choose to add some powdered kelp or quality sea salt into your soup right before drinking it for additional boost in nutrients.