Much like the hidden room in Arkham Asylum, I'm assuming it acts as a hint at where the next "Arkham" game would take place.
Which would be Gotham City itself. Along with the Scarecrow acting as the main villian.
Sadly, Rocksteady has stated they didn't have any plans to make another Batman game anytime soon, and wanted to do another IP.
Warner Bros. on the other hand, has already announced their plans for a 3rd Batman game, due out 2013/2014.
No idea who the developer will be.
So, we'll wait and see what they have in store.