4. Thomas Wayne (Flashpoint)
The Character: The classic Batman origin story, with one little twist. Instead of being the lone survivor, little Bruce Wayne is the only casualty on that fateful night. Thomas Wayne overpowers and beats the thug, and decides to wage war on crime for losing his son, becoming the Batman. His wife, Martha loses her sanity due to the sudden loss of her child, becoming the homicidal maniac Joker.
Well, at least he'll never grow up to become Ben Affleck.
The Game That Could Be: An older, meaner Batman with slower, but more devastating attacks could shake up the standard Arkham playing style a bit. Also, Batdad doesn't have Junior's no-kill policy, freely using lethal attacks and even guns in his fights. Ironically, despite him being more violent than the Bruce Wayne version, this Batman has a compelling reason to NOT kill The Joker. She is, after all, his wife and the last person he can call family. It would be Batman meets Snake Eater, with the protagonist having to fight the most important woman in his life. The plot has everything one could wish from a well-written, touching and memorable AAA title. Where is Paul Dini when you need him? The man should have his own Bat-Signal.
5. The Joker (The Batman: The Laughing Bat)
The Character: The Joker, feeling he doesn't want to be himself anymore, decides to be the Bat instead of trying to beat the Bat. Donning a crude imitation of the Batsuit, he starts fighting crime in his twisted, homicidal way. Noticing that something's missing, he knocks out the real Batman and injects him with a non-lethal version of the Joker toxin that would make Batman permanently insane. This way, the roles would be reversed, and the new Batman could have his own Joker to fight.
The Game That Could Be: There's actually two ways this could work nicely. One method is taking the exact scenario and making it a game. There could be two story campaigns. One could be a race against time as Batman, with the goals of finding a cure, defeating Bat-Joker and saving Gotham. The other could be ensuring Batman's insanity, locking him up in Arkham and turning Gotham into hell on Earth.
And the other way? Getting rid of the real Batman. It sounds extreme, sure, but riddle me this: What would happen if The Joker ever succeeded killing Batman? The guy is obsessed with the Dark Knight, there must be a Batman. Obviously, a replacement would be needed. But Joker's massive ego would insist he is the only one worthy. Also, Joker is only happy when Batman is close to him, and wearing the Batsuit is the closest Joker could ever hope for.
The experience would no doubt snap his mind in two, the struggle between Batman and Joker would continue in his brain. It could make a really interesting case of a moral choice system. Will he end up as an actual hero? Or will he taint the Batman legacy and use it for genocide? Or is it possible for him to just give up his obsession and walk away from the world of costumes and fighting? The more I think about it, the more I want to play this game. The game that will never exist. As the clown would say, the joke is on me.