I doubt many of us predicted a sequel to "The Cage" 55 years later. So, which other Trek episode or movie needs a sequel or prequel in Discovery? It'll never happen, but I'd like a prequel to the animated episode "Magicks of Megas Tu". Disco is right on the edge of fantasy and magic with their mycellial forest. Also a sequel to ENT: "Dear Doctor", or really any episode where they do something very questionable and we get to follow up on dire consequences a century later.