We're going to have a New 52 version of Stephanie Brown, and this is certainly cause for celebration not just for fans of that character, but also for Cass fans. Because we now know it's only a matter of time before Stephanie Brown's pal also returns to DC's ongoing storyline. But what we don't know is how. It's safe to say the new Steph won't be exactly the same as the old Steph, but I hope she feels "right" enough there's a continuity and her fans will be happy. While I'd argue there's not a single New 52 character that is exactly the same person he or she was before the reboot, Cass, unlike Steph, is a character completely mooted by the changes wrought by the New 52 in her mom Lady Shiva. In the previous continuity, Cass is the teenage daughter of a Lady Shiva with years of experience. The New 52 reduced Lady Shiva in age to a contemporary of Dick Grayson's, which means she's way too young to have a teenaged daughter. As a result, it will be difficult for any writer to establish Cass as a contemporary of Steph's and, like her, having been out there all this time, living her own little life before the DC spotlight falls upon her.
So while I'm on this topic, let's float some ideas about Cass. And to do so, I'm going to assume from the start any New 52 Cass will be re-set to a purer, primary state configuration, the way she was when we first met her all those years ago and before all those controversial changes.
One way to bring Cass back is...
Radically de-age the character. That is, if DC decides to keep Lady Shiva as her mother (more on that later). Unfortunately for Cass, we've already had Damian Wayne as the deadlier-than-you kid of the Bat-family and positioning Cass similarly would make her seem overly familiar and cover too much ground we've already been over. The uniqueness of a character and its impact on the overall story should be respected, so anything along those lines should remain Damian's area of operations even posthumously. Anything else would hardly be fair to fans of either character. Plus, this could turn Cass into a Hit Girl knock-off and I doubt anyone would let that pass... shall we say... unremarked.
Another possibility is...
Reveal Cass as some kind of artificially-accelerated test tube baby. This introduces some sci-fi elements to her backstory that could be explored, but tend to take the character in a direction apart from the parent-child issues that were so crucial to her conceptualization and development. I think Damian Wayne and all the Star Wars clone troopers have been there before, and comics are already replete with super soldiers of similar origin.
One more is...
Give her a different mom. This either means making her mother rather ordinary or creating yet another super bad-ass martial artist comparable to Lady Shiva. The latter seems like unnecessary character duplication, the same way a child Cass might come off as Damian Wayne redux. So they could always go with the former and make mom just some ordinary person who otherwise has the right qualities for producing someone like Cass. Whatever those may be.
I'd argue eliminating Lady Shiva from Cass' backstory is a possibility because father Cain and his relationship with Cass was essential to her character development in the early going, with Lady Shiva largely adding depth as the series progressed. While the writers hinted at it, we readers weren't exactly sure Lady Shiva was her mother, after all, until fairly late in the monthly's run. It was a cool reveal, nicely foreshadowed and set up, but all of those little clues could easily have been red herrings depending on how things played out.
Even so, the Cass versus Lady Shiva storyline was a major plot point in her series and their relationship tended to dominate the later issues. I think one of the few serious missteps the initial creative team on Batgirl made was giving Cass language skills too early in the run, but even so, this move did lead to the most spectacular confrontation in the entire series and the partial resolution of her death wish. The Cass-Lady Shiva pact and then fight-to-the-death wove a lot of the series' themes into what could have served as the overall story's climax. If the writers had killed her off at that point with Lady Shiva responsible, it would have been a perfect conclusion to a short but spectacular career.
Losing her Lady Shiva parentage would necessitate eliminating a lot of this good stuff and moving Cass along a different path. Which could also be a positive thing depending on the story quality, and largely what would happen anyway since it's New 52 Cass and not Classic Cass. While I'm obviously ambivalent about this aspect, one point in its favor is giving Cass a different mom means whoever writes her could spin out the mystery of her identity a long, long time and still maintain the essentialist approach I think is necessary to bringing back some version of Cass in a major way.
One thing they'll more than likely have to lose...
Cass' relationship with Barbara Gordon. While they'd interact, I don't see Barbara Batgirl as having a whole lot of free time these days to spend mentoring Cass. This was a major part of the Batgirl comic, but Barbara seems to have her hands full as it is and throwing Cass into the mix-- unless Gail Simone specifically requests it because it fits with her long-term storytelling plans-- just seems problematic at best. It might be better to slip Cass into a supporting role in one of the other Bat-books (especially since having her debut as the star of her own title seems about as likely as Marvel publishing it).
And about Cass' costumed identity...
As far as her identity as Batgirl goes, I'm fine with Cass coming back with no costume. She didn't jump into the black suit right away the first time around, either. With Barbara Gordon comfortably ensconced in that role and continuity kind of warped anyway, Cass could come back and have to prove herself all over again before earning her own code name, which could just as easily be Black Bat as anything. Or there could be a storyline where Barbara has to step back for whatever reason and Cass and Steph trade off the identity for short stints and we can all just pretend this reconciles past continuity with present. Black Bat gives her a reason to exist beside Barbara-Batgirl, and so I would be fine with that.
All I really expect from New 52 Cass is she be a silent and potentially deadly martial artist out to redeem her past. Give her a boldness that scares the bejeezus out of everyone around her so much so they worry she has a death wish. Keep her mysterious and shadowy for as long as possible and unpredictable, like a knife Batman tries to use that turns in his hand unexpectedly, cutting deep.
Oh, and don't try to "cute her up." Avoid the cutesy stuff altogether. I've harped on this a million times, it seems, but go after some of that Koike Kazuo feel, like with his amazing Lady Snowblood series. I think anyone writing a new version of Cass needs to read that until they have it practically memorized. Something a little tragic in there, the seasoning for a Cass version that attacks the palate like a deliciously strong spice, using just the right ingredients from her previous incarnation without resorting to re-heating leftovers.