Wonder Woman 3 should introduce Wonder Girl into the DCEU not only to expand the world but also to help overcome one of Wonder Woman 1984's issues. Wonder Woman/Diana Prince (Gal Gadot) has over the years become one of the most popular superheroes on the big screen. Through Batman v Superman: Dawn of Justice, Wonder Woman, as well as Diana's latest sequel and Zack Snyder's Justice League, she's been one of the DCEU's heavily featured characters. With the success of her films and appearances, Wonder Woman will be a character that won't be leaving the cinematic world anytime soon.
While Wonder Woman 1984 did get Warner Bros. to fast track the third installment, the follow-up to the 2017 movie had some struggle with Diana's arc. The untitled Wonder Woman 3 will reportedly be set in the present rather than in a past before most of other the big DCEU events. Gadot has been open about wanting to see future stories focus on her character in the present after having done two movies in a pre-DCEU era. The wait between Wonder Woman 1984 and the third film will supposedly be longer due to the global pandemic. The story details remain under wraps, but Wonder Woman 3 should use the next film to introduce more of Diana's comic mythology.
Wonder Woman and Wonder Woman 1984 introduced some of Diana's famous enemies including Ares, Cheetah, and Maxwell Lord. But for the next film, Patty Jenkins should start looking at bringing in some of the supporting players in Diana's comic history. One of them is Wonder Girl (a title that has been used by many) to add to Diana's character arc. In general, the DCEU could benefit from having more of the iconic teenage and young adult heroes to expand the individual Justice League characters' worlds. In the case of Wonder Girl, that title has been held by Donna Troy, Cassie Sandsmark, and recently Yara Flor, the latest heroine to take on that title after her DC Future State debut.
While adding any of the Wonder Girls to the DCEU would help flesh out the world, a partner would do a lot for Diana's journey. Since, by the present, she has been Wonder Woman for over 100 years in this universe, it would be organic to her arc to have a personal ally by her side. Not only would it let Wonder Woman 3 introduce another heroine to the DCEU, it lets Diana's growth be properly reflected. While his death was understandably still haunting her, Steve Trevor's return in 1984 complicated Diana's character development in the sequel, even after she renounced her wish. Steve has and always will have an important place in the Wonder Woman franchise. But Steve becomes a problem if he hinders Diana's character progression. It's sadly an issue reflected in Wonder Woman 1984.
That's why it's the right move forward by having Wonder Woman 3 happen in the present day and that's where a Wonder Girl would fit in perfectly. Having Diana mentor someone like Donna, Cassie, or Yara would be a refreshing and fun story for the next installment. It would also, generally, be good for the franchise to introduce another Amazon, especially someone like Wonder Girl who a strong bond with Diana. Whichever Wonder Girl it may be, it would be a strong move for Wonder Woman 3 and, most importantly, Diana Prince's DCEU saga.
