1. Red Hood
Here we are at last. All along, I've been talking about how important it is to maintain the atmosphere of a world where Bruce Wayne exists. You have to have his presence, and you need to have the mythology. That brings us to an entrant that HAS to have an appearance. I'd like to see him on a cliff-hanger season finale, myself. So who is this guy? Who is under the Red Hood? Well that depends. It could be any number of criminals, could be Jason Todd, It could be a pre-Joker, or even an entire gang. Having Red Hood would appease the fans. Don't give him the fish-tank appearance, but rather the Biker Helmet style. Give him a vest, tux, doesn't matter. His character doesn't really matter. What matters is that he exists, and fans would want to know who is under there. I'll tell you who it shouldn't be, it shouldn't be The Joker. It should just be some random recurring character from the show that fell into a life of crime. It was explained in comics that Joker got his outfit from the Falcone crew that had been using him. So go with the Falcone angle. It will work, and the fans will be happy.
Ideal Casting: Just give us a good tv, character actor, somebody that goes over well with audiences. My personal choice would be Greg Grunberg.