He's going to be a cop that admires Batman and helps him.
That's what I'm feeling. He's the Cop who gets the resistance on it's feet. It looks like he sees the chalk bat symbol from a kid in the trailer, and I think picks that up and uses it to spread the word (thinking back to No Mans Land and the Graffiti). Gets the cops back on their feet, or somehow helps Batman get out of his situation. He's a stand in for the Good People in Gotham City.
Yeah according to the earliest impressions from in-studio reviews of the script, the execs weren't happy with how it ends because it won't allow them to do a direct sequel with a new director and cast. Ending with Robin/Knightwing would not fit that sort of ending.
Personally I hope they don't kill off Batman.
I'm pulling for a "Everyone knows who Batman is" and he's not needed anymore. The City is saved and "fixed."