Bringing back the Watchmen is a big enough deal on its own, but it seems Doomsday Clock also managed to confirm a big piece of DC history.
Spoilers incoming for Doomsday Clock #2, so if you haven't read it you've been warned.
Doomsday Clock #2 brought the Watchmen and DC universes closer than ever, as Ozymandias and the new Rorschach journey to a new world to try and track down Doctor Manhattan. Manhattan presents the best chance to save their planet, which is close to obliterating itself with nuclear war, and their journey to find him takes them to Earth-0, the center of the DC Multiverse. It is there that they encounter a familiar location.
They end up crashlanding in the Owlship in an abandoned amusement park that you might recognize. Sure it has seen better days, but the Ferris Wheel, Tower, and Fun House are all dead ringers for the amusement park seen in Alan Moore and Brian Bolland's Batman: The Killing Joke.
In that story Joker terrorizes Jim Gordon psychologically in a variety of ways at the amusement park after shooting his daughter, crippling her in the process. When Rebirth started fans were unsure of what stories were still canon and which ones were left out, and Moore had described the original Killing Joke as not part of canon anyway, so there was significant doubt it would be included.
That changed though when Rebirth confirmed that there were 3 Jokers in the universe, one of which being Brian Bolland's version of the character. If that's the case then it makes sense that the events of Killing Joke actually happened in Rebirth, so having the amusement park featured is one more way for readers to see the connection.
There's always a chance that it was nothing more than a fun Easter Egg, but most things in Doomsday Clock are more than what they seem, so the odds are slim.
You can find the spoiler image above, and you can read it for yourself now, as Doomsday Clock #2 is in comic shops now.