If you go into the Manager's Office in the Steel Mill, and look on the ground by Harley Quinn's outfit, you'll find a positive pregnancy test.
Although I haven't found a
confirmed way to activate it yet, (though I suspect that it may be the final piece noted here) Harley can occasionally be heard singing in the end credits.
(Audio Starts at 0:50)
Harley Quinn said:
Shhhh... There, there.
Hush little baby, don't say a word, momma's
gonna kill for you the whole damn world.
There is a crib that you can find when you reach
Hush's hideout.
It can be seen briefly at 8:18, and then again at 10:10 - 10:21.
The crib in Hush's hideout is sitting under the words
"Dead End."
There's a Municipal Security Console right by it, whose password is,
Mocking Bird -
the song Harley is singing to her child.
While I currently suspect is that completing the Identity Thief mission prior to beating Joker in the Monarch Theater unlocks the soundbite of Harley in the credits, this is probably the second most interconnected Easter Egg in Arkham City, the first being
the Falcone Family, The Broker, & Scarecrow (click to view thread). This is loosely related, as Oracle suggests that Crane's the Fear Toxin may have been responsible for Bruce killing the people before the identity thief scheme is uncovered.
Much like that one, there's a single common theme:
good, old-fashioned revenge. Harley wants Revenge for Joker's death, Hush wants revenge for Bruce's dad saving his parents,
Scarecrow wants revenge for his defeat in Arkham Asylum. It's probably worth noting that there're some other individuals who likely want revenge due to the events in Arkham City though it's less explicitly stated, and they're more loosely connected through the side missions, in having ties to The Broker. The Riddler's final hideout was obtained from The Broker and he's always furious at being bested by Batman. There's also Zsasz, as his hideout was also obtained with the assistance of Mr. Fine (and contains the clue to the story section on it).
Then there's Azrael. The church has a stained glass window of St. Dumas on the opposide side of where you originally see Hush, and he leaves his message on the outside of where Hush was in the church. The idea of the foes that he bested in the Asylum & City staging a large scale revenge against him that spills into Gotham seems to fit with Azrael's message about ashes of Arkham coming to burn Gotham.
"Dark days are coming, Batman. The prophecy is coming true. You are the warrior who will close the gates of Hell. You are the one who will save this day and in doing so events will occur that you cannot stop. From the ashes of Arkham, the fires will rage and Gotham will burn. And you, you will burn too."
Either way, lots of interconnected Easter Eggs, and side missions give a pertty good idea on where the story could continue from here that I figured would be worth bringing up.