I usually hoard random bits of veg and protein and when I have enough I make like 5 gallons of soup at a time. Nothing is stopping you from adding more stuff later, when youre microwaving a serving of soup you can add some more steamed or roast veggies you have laying around.

It's also excellent with toast. Additionally you can dry leftover bread, crush it finely and add the breadcrumbs to the soup too.

Literally any leftovers you have can be incorporated into the soup one way or another.

I had some unappetizing lasagna so I chopped it up and threw it in my serving of soup, the cheesy sauce makes everything edible lol.