Well, Rachel Dawes couldn't have been Catwoman 'all along', as the identity presumably doesn't exist yet. Maybe she became Catwoman after the explosion or whatever... but frankly, I think the best bet here is a new character. Dawes was just a weak link in both films and I think she's served her purpose.
Johnny Depp as the Riddler is a great idea, IMHO. Heath Ledger's Joker is going to be a hard act to follow. I've got no doubt that Depp has the acting chops to be a great villain. Not that I've ever seen Depp as a villain in a film, but hey, he was great as the savage antihero of Sweeney Todd
. Scale that back quite a bit and I'm sure you'd get something interesting.
Personally I just think they should just bring Two-Face back. I think Eckhart had a lot more potential in him and falling off a ledge just wasn't epic confrontationish enough for me. Also, his return would clear Bruce's name and totally just ruin everything he and Gordon fought for.
I'd like to see him back (I had thought the film would leave him alive to be used more prominently in a sequel); but it'd be hard to explain away that apparent corpse. It'd be a lot easier to bring back Ras al-Ghul, and even easier to bring back Scarecrow (what happened to him, anyway?).
I always liked Bane, even though he looks like a Mexican Luche Libre Pro-Wrestler. He scared the crap out of me when I was a kid. Being a calm, cool, calculating, intellectual gentlemen one minute then transforming into a bloodthirsty, hulking, savage, brute the next.
An intellectual gentleman Mexican Pro-Wrestler sounds like a punchline to me rather than a villain. I know a lot of people were unhappy with the character's depiction in Batman & Robin
, but with a costume that ridiculous being a lumbering oaf is easier to swallow. Maybe if they overhauled the concept it could work.
Anyway, I definitely think the main villain(s) should be new characters, even if someone from the prior films returns in a supporting role. It keeps things fresh.