Thanks to some quotes from the likes of Cameron Monaghan and showrunner John Stephens, we can safely say that “Ruin” was not the end of Jeremiah’s story, although it’s probably not going to unfold how any of us could have guessed, and it could be part of a lead to The Joker.
Admittedly, the story has always been that the JOKER Joker would never appear on Gotham, as Gotham is an origin story for Batman’s iconic characters, and The Joker technically didn’t need to emerge until Bruce was already Batman.