Thing about Harley Quinn is she's usless w/o Joker.
The Joker briefly alluded to
being married during one of his scar stories and if that's to be believed then its certainly plausible. I really think a darker, grungier version of Harley would look incredible, I can imagine her wearing a porcelain mask
to cover the scars or something of the sort, a la a French mime.
The thing about the Riddler is that it he can't be too similar to the Joker. Also, I think that if he appeared in the next film everyone would be comparing him to the joker because they are similar.
That's why I thought a version of the Riddler more akin to the animated series would be better. In that he was cool and collected, and despite being egotistical he was more cunning than outright anarchic and psychopathic.