Wonder Woman isn't a Black Widow type character whom you can introduce as a supporting player in another hero's sequel.
She's as important to the League as Captain America is to The Avengers, and she needs her own fish-out-of-water film.
Joss Whedon knew it, that's why he took a Wonder Woman project to Warners, only for them to turn him down. The studio needs to stop kicking themselves for that mistake, and put a Diana Prince movie into pre-production as soon as possible.
A Wonder Woman film is an essential lead-in to The Justice League, it won't work without it.