@supbatz: I agree with all your points. Joker is incapable of showing affection to anyone but he has tried to learn all the emotions he cannot understand through Harley, and that's interesting. I think that Mark Hamill said something similar once in an interview.
I hope we could see them working together in the future, they collaborations are always memorable
@leah8954: Yeah, that's what bothers me beyond reason; the writers have to respect the canon because that's what made us love the characters in the first place.
Regarding Harley, they cannot make her forget about the Joker with other guys! That would definitely destroy the essence of the character and a lot of the fans would be very pissed off. The Harley we all know can't live without the Joker, he is always in her mind and she would never betray him. I remember how upset the fans were when Harley and Deadshot kissed in the Suicide Squad. People were really, really angry.
By the way, I hated Harley in the Suicide Squad, she was completely out of character there.
It would be interesting to see how Harley could react if she encounters Duela Dent. Two clowns very attached to the Joker's figure :P
Also, I consider myself a fan of the Joker-Harley moments, the relationship they share is what attracted me the most when I saw Harley for the first time in The Animated Series. She is the only gal in the world that has seen the Joker at his worst, the only gal that has seen the infamous Clown Prince of Crime behind his smile and that's fascinating.
I would like to see the Joker appearing eventually in the series. If someone appears in the picture trying to woo Harley, I don't think that The Joker is gonna like it. An act of jealousy and doing something nice (for once!) to take her back will put a smile on the faces of a lot of fans like me ;)