Also makes me think it wouldn't hurt to add chopped or shredded chicken to French onion soup to make it heartier. May be heresy to mess with a classic, but sounds like a plan to me.
You said it needed something. The soup recipe I use has a splash of white wine, some Worcestershire and a dab of sugar. Maybe one or some of those would help? I really like the soup recipe I use; it's very rich tasting for a broth-based soup.