Okay, so, from the looks of things i *have* to have an Evil!Cass.
Note I said an.
The way I see it, clones are clearly the way to go here. Now, the secret message within 52 (DC's series explaining what happened in the lost year) was that alternate worlds still exist right?
So the idea is that our Evil!Cass comes from one of these alternate worlds, and was supposed to be sent to a "The One" style prison world, but managed to secretly wrangle it so that she ended up here. She then hires some psychic that she knew from her old world, and then ambushes and mindfucks Cassie over into thinking that she's an ordinary university student (or high school, I don't remember how old exactly cassie is) they enroll her up in some out of the way Gotham uni/boarding school. Evil!Cass then takes Mindfuck!Cass's place as batgirl.
Meanwhile Nyssa, who currently runs the league of assassins is trying to get Cassie to sign up as their leader (remember?) so in order to sweeten the pot she digs up Steph and exposes her to the lazarus pit.
Unfortunately Steph comes back a little psychotic, manages to permakill Nyssa somehow and escapes.
The league of Assasins find out that Nyssa's dead, and decide to immediately offer Evil!Cass the job as leader. Since Evil!Cass is beginning to get worried that people might realise that she's not the real Batgirl she accepts.
Meanwhile Steph's running around trying to figure out what's going on. It doesn't help that because of the Lazarus Pit she's getting psychotic black out episodes. So Psycho!Steph goes to Gotham, except since this is in the middle of 52, Batman and Robin are off on holiday, and Evil!Cass is out with the League.
(She can't go to Bruce for help remember because she doesn't know that Bruce is Batman.) Somehow she does manage to meet with Mindfuck!Cass and manages to convince her that she's Batgirl. The psychics hack work begins to unravel at this pojnts, but there's still a lot of her memories she doesn't remember (she still knows how to fight, but she doesn't remember *how* she knows how to fight for example.)
So now you have a Mindfuck!Cass travelling with a Psycho!Steph trying to find the psychic who did this to her or Evil!Cass. But she's still not sure whether or not even if she's really is Batgirl or not (especially when you consider the only person telling her that she is happens to keep having these psychotic episodes.) Spend the rest of the comic jumping between Mindfuck!CAss and Psycho!Steph, and with Evil!Cass.
They find psychic, coerce him into undoing his hack work (it's not hard, the guy in question isn't so much evil, as amoral and thinks with his dick to much. He's basically DC's equivalent to the Purple Man)
They then track down Evil!Cass.
Showdowns ensue.
Also make sure to intersperse it with flashbacks to cass's post Cain child/teenhood enough to try and dissuade some hack taking your work later on and adding Rape to her backstory.
Well. That's my idea.
