>>113301107I basically thought about this myself when I saw an issue of some DC comic in BAM that had Harley being a Green Lantern and wondered what it would be like if she joined the Bat-Family. Only my version was pornographic which showed the sex between Harley and Batman and it was one of those stories that had a lot of story with the sex as the backdrop. She even ends up raping Nightwing, Robin, and a crippled Batgirl due to misunderstandings before Batgirl brings her back to reality and she reconciles with Batman and everyone. Would be cool if a porn artist ever tried to take a crack at this.
I've also been curious about another thing, have they ever done a version of what the kid of Batman and Harley would be like? I know Batman has kids with some of the leading ladies in his life, but not all of them. Off the top of my head:
Batman + Catwoman = Huntress
Batman + Talia = Damien
Batman + Harley = ???
Batman + Poison Ivy = ???
Batman + Lady Shiva = ???
I got the last one from here since I didn't know a lot myself.
https://thegothamarchives.com/female-batman-characters/ 
