Mutations may make things worse, but not likely. Mutations are more likely to make things better. The true nightmare situation would be this: a virus which spreads easily (probably upper respiratory), and results in a very mild, low/no symptom disease, where people are contagious, mobile, and not visibly sick, for a long period of time. In this manner, the virus spreads everywhere. But in a true nightmare situation, people would not develop long-term immunity, and the virus would be able to get into the lower respiratory system and other organs upon reinfection. In this scenario, the virus would never really go away, since it would just bounce around populations indefinitely, but it would be a Russian roulette situation in that any particular reinfection would contain significant chance of a much more serious case.
Mutations aren’t what to worry about. More serious reinfections are.
Spanish flu of 1918 grew Notre virulent upon mutation, in the fall.
I’ve seen published studies that a single amino acid substitution at the wrong point with novel Coronavirus, will greatly increase its affinity for the ACE2 receptor...