Farscape was one of the earliest shows to play with continuity and to explore shaking things up -- maybe not to the degree that you'd expect from a 2010s or 2020s show, but it was very surprising for a show that ran from 1999 to 2003.

It started with a lot of stand-alone episodes but moved towards continuity as it progressed, and kept moving some of the antagonists around into different positions that you wouldn't expect. It also toyed with some of the protagonists' sanity as they seemed to be experiencing PTSD or something similar as a result of all their grim experiences, something you didn't see on other shows -- though of course, in one case, the loss of sanity turned out to be caused by another, more interesting continuity element entirely.