If pertussis is left untreated, how likely is it for the disease to cause any other problems?

Depends on variables: Pertussis is most deadly to infants under 4 months of age and less so to older kids. In adolescents or adults it produces the "100 days cough" or a seemingly endless bronchitis. Untreated it may self heal while you contaminate the environment & spread it directly or indirectly to more vulnerable patients. If you have emphysema or other lung issues it can aggravate the disability involved.