All soups start with a good base: To get the best flavour, you need to sweat onions, celery, carrots, chopped very finely, until just turning little caramelized. Not brown. This takes time but is definitely worth it.
Don't even think of adding liquid until you've done this.
If you are going zero waste, note that if you finely slice up celery leaves these will cook down to nothing and they are packed with flavour. I never waste celery leaves.
Add a bay leaf too (I grow a small bay tree in a pot beside my back door - makes a brilliant xmas present for a keen cook!), a really big pinch of dried thyme, salt and pepper.
Then you can add whatever you want - but be careful with sweet ingredients like parsnips as they can quickly make the soup sickly.
If you want to a thicker soup, add a few finely chopped potatoes when you sweat the veg. I agree with pp, Smash flakes do a brilliant job instead (my family is snobby and not generally keen on Smash but love my soup and no idea it is in there).
If I have them lying around, chuck in remnants of pots salsa, leftover baked beans, etc.
Also, saving up the broken bits from bags of dried pasta in a "soup jar" is a good idea - you can put the pasta in towards the end and it adds substance.
A dash of Worcestershire sauce, a squeeze of tomato puree or a pinch of sugar to balance things out if the soup hasnt turned out quite right, can all help .