My possible story for Donna is much darker, and perhaps not in keeping with the cartoon’s general mood. For one thing, my re-imagining of Wonder Girl borrows from several of her different incarnations. As with your scenario, this arc would be spread over five or six episodes. So for your entertainment…
Donna Sandhurst is a seemingly normal 18-year old perspective college student. She goes to the San Francisco area to check out UC Berkley and/or Stanford. (Jump City is part of the San Francisco metropolitan area in my head canon.) While there, she’s thrust into a traumatic situation, like a bus crash, where she displays extraordinary strength, reflexes, and other Amazon-style powers, which save several bystanders. She’s always been athletically gifted, and was an accomplished mixed martial arts student at her high school. But suddenly she had the strength of 10, which freaked her out. Strongly suspecting that she’s some type of meta-human, the Titans come on the scene and spirit her to the relative safety of the tower.
Back at the tower, Donna is in Raven’s safe room trying to come to terms with what just happened to her. The first question the Titans have is what kind of meta-human is this? Research finds a historical meta-human who was active in the 1940’s, and which had similar physical abilities: Wonder Woman. Wonder Woman, also known as Diana Prikapas (Greek for Prince) hasn’t been seen since 1946. The assumption has long been that she returned to her world of origin after the Axis was defeated. (This is actually true.) But the Titans are now wondering if she had any children, or if another Amazon has come to Earth. The debate is creating more questions than answers. Frustrated, they take a break by ordering pizza and wings.
A short time later, Beast Boy goes to answer the door, figuring it’s the food delivery. Cyborg and Nightwing both agree that this historical meta-human is their best lead, so they decide to find out whatever they can about her. Beast Boy then enters the room, looking pale-faced, points behind him and says “Why don’t we just talk to her.” A tall, imposing black-haired woman emerges from the shadows. She’s in street clothes, but it’s clearly Diana. Cyborg even runs a facial recognition program against a period photo of Wonder Woman, and it’s a match. She doesn’t look at all different! Raven responds with her classic, understated, “Woah!”
At any rate, Diana wants to speak with Donna. Immediately.
At this point some exposition will tell Donna’s backstory. Her mother is an Amazon, who defected to Earth in the mid 1980’s. She eventually married a human man, and had a daughter. The Amazons have been discreetly watching Donna for quite some time, wondering if she would exhibit Amazon powers. When she did not do so during puberty (which is the norm) they figured she would remain a normal human, and so kept their distance. The theory now is that the trauma of the crash triggered something in Donna that caused her powers to suddenly surface It is eventually revealed that the crash was set up by Brother Blood. Brother Blood somehow learned of Donna’s origins, and attempted to jump start her powers by creating an extreme situation for her to deal with. It’s also possible that he used some sort of bio-tech to awaken Donna’s powers. Regardless, this part of his plan actually worked. Brother Blood had planned to capture and recruit Donna, but the Titans got to her first.
Eventually, and after much difficulty, Donna comes to terms with her origin, and agrees to be trained in the Amazon way by Diana. She learns how to use the trademark lasso and deflective bracers, while also enhancing and expanding the human-style martial arts training she had as a teen. Diana also starts teaching her the telepathic/telekinetic skills of Amazons, which would include limited flight. But that will take some time for her to perfect.
At the end of the arc, Kori asks Donna, “So, are you now the new Wonder Woman?” Donna will give an embarrassed chuckle before answering. “I’m nowhere near the warrior Diana is. I’m not ready to take on her title. For now, call me Wonder Girl.”
Or something like that. YMMV.
Make sure to include an explanation for why we haven’t seen Donna before, especially since Robin was networking with the other meta-humans around the world in season 5. But you’ve already set up some of that, so the additional details should be easy.
Another possible line for Beast Boy would be “OK, didn’t see that coming…” In my head-cannon “season 6,” Beast Boy has an experience that causes him to “click,” and he starts to noticeably mature. But that’s another story for another time.
Regarding the rivalry between Donna and Kori, I wouldn’t let that run too long, since the writers jerked the audience around for a while regarding those two. At some point I would have an exasperated Donna say to Dick and Kori, “Are you two a couple or not?!? The U.E.T. in this room is getting painful!” The other Titans would respond as one might expect. Changeling: “Dudes, she’s got a point.” Raven: “Yeah, what’s up with you guys?” Cyborg: “I thought you two settled this back in Tokyo.”
I agree that Kid Flash should remain a recurring guest star, at least in the short run. I’m thinking the transition from Robin to Nightwing is in part related to the appearance of Damian Wayne, and encouragement from Bruce. Since Dick is making his own mark, and is no one’s sidekick anymore, he should use a new moniker to reflect that.