We've known for a while that there would be Harley Quinn DLC for Batman: Arkham Knight at launch. That was announced almost alongside the game.
What we didn't know was exactly what it was. Thanks to an Amazon listing (via IGN), we've gotten our first hints and it could be a little more substantial than we might've thought. The product description reveals:
For the first time in the Arkham series take on the role of Joker's psychotic side-kick Harley Quinn and wreak havoc inside the Blüdhaven Police Department. With this exclusive Story Pack experience events in the lead-up to Arkham Knight as you infiltrate Blüdhaven to rescue your partner in crime: Poison Ivy. Delve deep into Harley's demented nature and utilize her devastating baseball bat to smash, crash and bash your way to a friend very much in need.
This DLC is Amazon exclusive for the moment, so you'll have to get your Harley fix by pre-ordering from the online retailer.
Besides one or two throwaway lines in Asylum, this may be the first time we've seen the friendship of Quinn and Ivy explored in the Arkhamverse. If nothing else, that is pretty cool.