Try including some mole powder or paste next time. It usually has peppers, raisins, chocolate and spices plus other flavors like peanut butter. It depends on the version you buy.
In any case, dark chocolate and/or instant coffee add an interesting bitterness. A good English bitter or stout has the same effect.
I think the most critical ingredient is FRESH cumin. I like to grind the seeds just before putting them in the chili, and add them at various times during the boil. I always add some powdered New Mexico chili and fresh cumin about 5 minutes before taking the chili off the heat.