While it remains semi-speculative at this point, an image revealed on the official Arkham Knight site seems to give a pretty good indication as to the contents of the second challenge pack. Based on the picture, it appears that there will be five different maps included. One for Batman, one for Catwoman, one for Nightwing, one for the Batmobile and a fifth for Robin/Nightwing dual play.
Interestingly enough, the site also states that there will be six challenges, which may mean that another character will have two challenge maps to themselves.
The Arkham Knight challenge pack #2 is expected to drop sometime later this month.