We have had a few threads on this topic a lone I believe but now we have official confirmation as to what this movie actually is.
In essence, its a good excuse to use and bring back Steve Trevor and potentially keep him alive (depending if his character survives this movie) for future movies and completely retcon BvS.
SOURCE
But Wonder Woman 1984 producer Charles Roven — the veteran producer behind Justice League, Batman v Superman: Dawn of Justice, Suicide Squad, the Dark Knight trilogy and, yes, Wonder Woman — would like those fans to know that they are wrong. Wonder Woman 1984 is by no means a sequel to the previous Wonder Woman installment. And according to him, Jenkins herself would like to avoid any perception of the S word. "She was just determined that this movie should be the next iteration of Wonder Woman but not a sequel," Roven tells Vulture. "And she's definitely delivering on that. It's a completely different time frame and you'll get a sense of what Diana-slash–Wonder Woman had been doing in the intervening years. But it's a completely different story that we're telling. Even though it'll have a lot of the same emotional things, a lot of humor, a lot of brave action. Tugs at the heart strings as well."
In essence, its a good excuse to use and bring back Steve Trevor and potentially keep him alive (depending if his character survives this movie) for future movies and completely retcon BvS.
SOURCE