For those of us who are familiar with the comics, this is a loaded question.
A little background to bring people up to speed. Back in the Golden Age of comics, Wonder Woman would have adventures with her younger selves, Wonder Girl and Wonder Tot. The comics were clear that they were Wonder Woman at previous stages of her life. Editorial forgot that little fact, and thus was Wonder Girl became a full-fledged character. Really.
Unfortunately the character has suffered from the sketchy background. Many attempts have been made over the past 40+ years to give her a background rather than ‘teen Wonder Woman’ with mixed results. Donna has been tossed into limbo with the DC reboot.
So, the original question remains. Who is Donna Troy? In my series, Donna’s still an orphan, in her teens. Her mom is Etta, rather than being a foundling discovered by Diana. She would quickly develop a sort of hero worship for Diana, leading to some friction between Diana and Etta. She also would be the one to play off Diana’s lack of modern culture, allowing for some humor. If Donna is working on school work, and says “It’s Greek to me.” Diana would be the one to straight-faced say, “No, that appears to be English.” Donna does not have (initially) any superhuman powers, though the idea of her ‘borrowing’ artifacts to give her powers for an episode are not out of reach (this would also be a nod to Cassie Sandsmark’s original origin).
So who plays Donna Troy? I’ve two people in mind, given that the role (and actress) would invariably grow older. If we want a younger Donna, I’d cast Kiara Glasco. This young actress blew me away in Copper, and I’d love to see her in a more heroic character. For a more mature Donna (late teens) I’d cast Hanna Leigh. This 17 year old caught my attention in an episode of Body of Proof, and her ‘innocent to psycho in 10 seconds’ act made me want to see her in a regular role. With Ms. Glasco, I’d likely tease her getting superpowers (including the Cassie Sandsmark reference above) and stretch out the tease over a few seasons, to let the actress mature. With Ms. Leigh, I’d go with her being exposed to the Amazon’s Purple ray at the beginning of season 2, and ‘officially’ make her Wonder Girl in season 3.