Doomsday Clock #2 has just been confirmed as a very important book in the canon of the Dark Knight and DC as a whole.
In this particular issue, Ozymandias and Rorschach track down Dr. Manhattan to Earth 0, where they land in the same amusement park the Joker carried out the infamous heinous acts in The Killing Joke, as can be seen below:
Alan Moore, the book’s writer, has said in the past that he believed that the book was not part of canon. This changed when bigwigs at DC confirmed, with the Rebirth launch, that there 3 Jokers, including the one created by Moore and Brian Bollard. Time will tell if we get to see any characters from that story in the near future, but it would certainly be interesting to see what transpires after that traumatic event that still makes fans cringe.