I think Riddler is by far the best choice for the third Batman villain. And, Samsara, I really like your take on it.
But I can't agree with you on Catwoman or Joker. I always thought Catwoman was a terrible idea for a character. Activist-nazi's only hold signs, rarely do they actually act on their beliefs. And beyond being a woman in a cat suit, what's interesting about her? In the animated series - nothing. And to be fair that's my only reference for the character. Maybe she's developed better elsewhere. The Joker should just not be seen or heard in the third film out of respect for Ledger. And, to be honest, I want to be able to watch Batman Three without ever being reminded of what could have been if Ledger's Joker had another film. It's too sad in too many ways.
The most important part of Batman Three is going to be getting the story right. Where is Bruce Wayne as a character at the end of all this? After Joker and Two-Face, how much further can Batman be pushed? This is why Riddler might be good as a villain. The Joker created an unstoppable force vs. immovable object relationship. Two Face asked if Batman's value were really right in the first place. But The Ridder is smart enough to defeat Batman by actually unmasking him. The Joker made Batman's brute force look useless at times, but The Ridder wouldn't even let Batman lay a hand on him. But even still, this are merely expansions on past themes.
The villains in Batman Begins, while very good, were comic book villains. The villains in The Dark Knight actually took Bruce's soul head on. What else is there?
Just no Robin, and I have hope Nolan, Nolan, and Goyer can make it work.