Does anyone see that Cass could be up to something, setting up a storyline where she goes undercover for a year or so to infiltrate some organization or other? She shows up for a few months as a new mystery crimefighter who is eventually unmasked and reclaims the Batgirl uniform, while Stephanie goes back to being Spoiler (or does the undercover gambit herself?)
Almost everything has all been done before in fiction, the details change but storytelling has its own rules and limitations. Writers move the characters through various processes over and over.