You need more of a reason to play as her.. ok i thought the picture would be enough for you but if you insist. Harley is a superb gymnast and is very quick and agile, not the best hand to hand combatant but still good and with weapons and fire arms its a done deal. Would be funny if they gave her a mallet as a gadget though, her fighting style wouldnt be hard to implement and gadgets all with hilarious perks while watching her fight and take down baddies would be to awesome for words.
Alt costumes for Harley Quinn:
The Son of the Bat should make his debut in a video game, even if its for challenge maps and non-cannon it would be great to see this lil master fighter appear. He's a powerhouse for being so young (Did you forget who his daddy is?) Damian's fighting style comes with years of training shouldnt make him a hard character to give a moveset and gadgets to. Plus being the new Robin does have its perks, how about they give him that sword on his back in the picture and work there way around that.
Alt Costumes for Damian:
Barbara Gordon aka Batgirl:
What better way to show how she got her start then playing as her before her unfourtunate accident, being trained by Richard Dragon and Batman himself mixed with her own self style of fighting crime she would be the perfect person to play as in a series of challenge maps.
Alt costumes for Batgirl: