My hopes would be for a simple, logical explanation and a very close relationship between Donna and Queen Hippolyta, Diana and the Amazons. Ideally, it would be as basic as possible - Donna is the sole survivor of a shipwreck, washed up on the shores of Themyscira and was adopted by Queen Hippolyta and become Diana's younger sister. Amazon training and maybe some Amazon technology or magical weapons give Donna super-powers to rival Diana (although maybe not quite as powerful). A future storyline could explain that Donna was saved from drowning / dying and guided to Themyscira by the Titans because they sensed some cosmic destiny for her as a future hero and sought to ensure she received the best training available to achieve that goal. I know all of that doesn't fit with Rebirth, but I'd be happy for them to do some hand waving to make it fit with Rebirth.
My fears would be that Donna receives yet another convoluted, overly complicated and unnecessarily ridiculous origin story which will be rebooted yet again at some point in the future. And that her ridiculous origin story somehow turns her into a man-hating, blood-thirsty, irrational, aggravating character who has no close ties with Diana, Queen Hippolyta or the Amazons. And that she's forced to run around in some kind of skimpy bathing suit costume with high heels and dramatic cleavage while constantly carrying a sword which she uses to cleave the heads off her enemies.