Gotham Girl is an interesting idea and would provide some new narrative options. I didnt read that arc (trade waiting a lot of stuff these days) but I followed what was going on.
Black Lightning would be interesting to see, though I dont know how well he'd fit into the "Bat bootcamp" concept.
Everyone here will hate the idea, but what about Harley Quinn? She's been doing the anti-hero thing for a few years now and she's worked with several heroes, including Batman (sort of). I could see Bruce deciding to bring her under his wing in the hopes of real redemption. If nothing else, I'd like to see her brought in the next time Bruce goes up against the Joker. It makes a lot of sense that Bruce would bring her in for that, even if he only intended to get some perspective from her.