Add different modifiers that are toggled at the start of a endless run that changes some of the way you play. Some of the ideas I had were:
Symptom Randomiser: All symptoms are remixed so some "Sick" Symptoms would actually mean "Infected." And wise versa. I think it would be funny to find a person with freckles and suddenly be like "Op, there's a zombie."
All symptoms known: You start the run with knowing what symptoms there are and what they mean
Wolves in Heavy clothing: Allows you to use tools on zone guards and scientists every day they have a chance of being an infected
etc...