If we had to have another Batman character, these are characters that I think will bring new stuff to the table:
Poison Ivy: Control of plant life will ensure that she'll have zoning abilities along with vines that allow grappling and stasis type moves. Using poisons and pheromones, she'll also be able to buff and debuff causing all kinds of status effects. Possibly "teleport" options by going into the ground and "sprouting" up from a different location.
Scarecrow: Will have normals focused around the use if his sickle. His trait could possibly be his fear gas that makes the stage "nightmarish" and may also transform scarecrow himself into a creepier form kinda like a "stance change".
Penguin: Small hitbox alone will make him unique from the cast. His moveset will focus heavily on his gimmicky umbrella which can shoot bullets, extends blades, be used as a flamethrower, spew gas and acid, allow him to fly/float, be used as a shield, and more.
Ra's al Ghul: What would make him unique is that he'll be a disciplined sword fighter, exhibiting more grace and flourishes as he swings around his blade.
Killer Croc: He'll probably have moves that bring him on all fours, along with biting normals and use of his tail. Basically a more rush down oriented "big character".
Riddler: What he lacks in skill, he'll make up for in demeanor. He'll basically have moves that have the same start up, but different effects and hit boxes keeping the opponent guessing. His trait though is a bit if a puzzle to think up...
Those are my thoughts on Batman chars I'm most willing to accept.
