Harley isn't half as good w/o Joker (although it was always neat when she teamed up w/Poison Ivy)...the tension between the two is what really made her character.

The Riddler was always a half-assed member of Batman's Rouge's Gallery...

I'd much rather see Mr. Freeze. He always had one of the best background stories of his villains and was one of the ones who could actually kill him. Besides, we need to wash that horrible Arnold Freeze taste out of our mouths.