I would say do as T&S has said , just throw in some puree or mashed veggies that you like.
Whenever I make my own tomato sauce i thrown in some other stuff just to see what would happen.
That has been things like (not all at once , but sometimes combinations of 2 or more); capsicum, parsnip, carrot, potato, celery, lentils, various beans, mushed peas, cauliflower, sweet potato, swedes, broccoli, turnips, beetroot, anything thats around at the time.
Stuff like onion and garlic are a given.
You can also add parsley, basil and oregano.
They all add to the colour, thickness and consistency a bit and sometimes the taste(which can be a good thing) but most of the more benign flavored veggies go unnoticed.
I would say experiment and have fun. make a small amount first and learn as you go. Its the best way .
I find most good cooks don't usually have defined measurements and recipes, that is from experimentation - the best way to learn.
Its all edible (but then again i eat anything and everything)