Travis Lee Ratcliff has a nice video essay exploring what makes a prequel successful. In short: characters and events in the prequel need to have meaning beyond their original context. Many prequels forget this (including Better Call Saul, at times) but I think Saul has really cleared the bar for creating something new and satisfying out of an existing world.
[Thanks to Søren from the Slackfilmcast for bringing this essay to my attention]