The opened ended nature of the city itself and the game's ending, complete with a return for the post game, means story DLC for this game could be really elaborate.
While I wouldn't mind more Batman or Catwoman story arcs, I'd really love a Nightwing or Robin story. I really like their designs after playing their Riddler's Challenge selves.
But then, while the art style in the game isn't to everyone's liking, I truly love it, and I love Rocksteady's spin on the Batman designs.
It's kinda like, a bit of the gritty realism of Christopher Nolan Batman, but still a fantasy universe where there's sorcery, super science, and so forth. Their designers tried hard to make all the costumes and such look plausible and constructed out of real materials, even practical to use. Or at least over the top science punk. Freeze's armor is goddamn epic.