Re: Ant-Man and Dr. Strange part of Marvel Cinematic Universe phase 3
And part of the fun of superhero comics is that they're a total melange of genres, everything from pulp crime fiction to space opera to fantasy to horror to Westerns to soap opera to comedy. If the Justice League (at least in the DCAU) can include Martian Manhunter, Vigilante, Shining Knight, Orion, the Demon, and Adam Strange as well as Batman and Steel, then it's not that great a stretch to throw in Zatanna and Dr. Fate as well.
It's also not a stretch to include Dr. Fate in the JLA, since he was a founding member of the JSA in 1940.