I've had this idea for ages about someone who goes back to like 1000 AD and tries to introduce germ theory, disinfecting surfaces and medical devices with alcohol, etc. The conflict comes from them having to overcome medieval institutions being resistant to change, starting with them naively writing a treatise on what is currently considered medical common sense but getting rebuked for apparently not knowing what they're talking about.
Naturally, the main character wears a cute version of a plague doctor outfit.