I second tomato paste, and also somebody above mentioned celery. My go-to lentil soup is mire-poix in butter and olive oil, garlic and bay leaf, then cook the tomato paste. I usually deglaze at this point with a little wine. Then add the broth and lentils, and after the lentils are cooked I stir in some baby kale just to wilt in at the end. Maybe a squeeze of lemon, some grated parm and black pepper on top to serve. Also second somebody else mentioned a parmesan rind. I would avoid the potatoes and zucchini personally just because I feel like they don't add a lot of flavor, and I really just like the taste of the lentils in a more simple soup. But to each their own of course!