4. Power Girl
I know that Wonder Woman and Superman are heavy hitters, but someday I could see one of them stepping down or being defeated. Power Girl is basically at the same level as Superman and is even one of his cousins in some continuities. If they ever wanted to add another super powerful female hero and didn’t want to go the Supergirl route then Power Girl could be a great option. Her origins have changed so many times that the movie writers could go many different directions with it.
Her personality is the biggest thing that I think she could bring to the team that they are missing. I see her adventurous and easy-going spirit being similar to that of the MCU‘s version of Thor. While Superman moped his way through Man of Steel, Power Girl would have rolled up her sleeves and slugged it out with Zod and Faora with a smile on her face. The recent Justice League trailers have claimed that Superman-inspired people to be heroes and Power Girl could become one of those people. A student of the only other person with the same level of power.