Without giving away details, both Catwoman and Robin have unique mini boss fights that involve characters Batman doesn't face in his campaign. Do you suppose Nightwing will have his own unique villain encounter?
#1 quarreler. Undefeated in all my quarrels. Just try me, pilgrims!
Killer Croc perhaps? All the villains faced by Robin and Catwoman did appear in Arkham City's main story, and the only unfought villains that show up were "Identity Thief," Croc and Bane. I suspect "Identity thief" wouldn't be used for obvious reasons if you've finished that side-mission. Bane got a prominent side-mission so it COULD be Croc, but it's most likely no one.
"I'm going to go do British things, like drink cups of tea and eat bangers and mash, that sort of thing...because I'm British"