Why didn't they retcon Power Girl into being Supergirl in her early years as Pre-Crisis elements came back Post-Crisis? You can have her do the Silver Age/"early years" of Superman's career, being innocent and friendly like Silver Age Supergirl was then, but growing into the confident and self-aware Peegee we know and love. She can become a Nightwing/older sibling equivalent to the Super-family and give it some needed codename and costume variety. She can even keep her Earth-2/"from another universe" status once they bring back the native mainstream Kara, and make a show she's still truly family to Superman as much as the native-universe equivalent naturally is.

I'll point out I'm aware of the "Superman as sole survivor" stuff Post-Crisis which is why I pointed out they can keep the alt-universe stuff as a point as they do stupid origins for her Post-Crisis as in reality till they do this retcon. I also know since New52 she had been a Supergirl but I'm talking a mainstream Peegee, not the dumpster fire of continuity New52 begat even by DC standards.