The Mr. Freeze DLC is expected to be a story mission just like Harley Quinn's Revenge in Batman Arkham City. While it is yet to be confirmed for release, here is what is expected to happen in the DLC story mission:
Mr. Freeze will be the central villain
Freeze will have henchmen helping him carry out his plan
Ferris Boyle will be targeted by the villain for the accident that made Victor Fries Mr. Freeze
Nora Fries might appear
Gotham's Humanitarian Industrialist of the Year Ceremony is hinted to be held at Wayne Manor as sources say
This mission may take place in the Dark Knight's third year of fighting
Deviations on the plot and setting on the mission from the Batman TAS episode Heart of Ice are expected
and lastly and which is not yet officially announced:
Barbara Gordon may become playable in this story mission as Batgirl for the first time
otherwise it's gonna end up being Dick Grayson/Robin and that Barbara never becomes Batgirl in the arkham games at all
to find out, be sure to keep your eyes open for any teasers that come up on the DLC