But Wonder Woman's themes were so pronounced, that it's easy to at least write something the day after. Obviously, spoilers below (in fact, this really won't make much sense without seeing the movie.)
The DCU movies explore "What does a superhero (or a god) MEAN to the world?" Marvel movies (with the muddled exception of Cap) are mostly "What if this witty person got some power, what cool things would they do with it?" (See my post on Superheroes and Art.) But from the first conflicts of Man of Steel, Snyder and other auteurs are dealing with "How does the world react to... The Superman?" There's shock, and terror, and over-dependent hope. It's very serious, abstract, archetypical stuff. (See ignorance and innocence in Batman vs Superman.)
(You can include Batman in this grouping, mostly in the negative sense. Every single DCU movie, including the Dark Knight trilogy, are pretty critical of Batman and his various failures to be as idealistic as someone like Clark Kent. He's just a brutal dude in a suit.)
WW continues this mythological soul-searching by saying "Okay, how does a godlike being even make a difference in the giant, chaotic, suffering world?" It's actually very similar to Strong Female Protagonist, which pithily asks the same questions with "What are you going to do, punch poverty in the face?"
This is framed by the running argument between the leads, Steven and Diana. In the midst of World War One, Diana asks questions like "take me to the war", and Steven is just dumbfounded. The war is everywhere. It spans the globe. As he says, just one front they are fighting on is 400 miles long. There's no one place to go to.
Visually, they demonstrate this by forcing Diana to walk past suffering people, repeatedly watching injuries and problems that she could help with, but there are just far too many for her attention. All the need is just overwhelming. We get this in the first shots of London, and on the docks as wounded soldiers come home, and seeing families and soldiers evacuate a town, and finally in the trenches. What can one person, even an all-powerful person, really do?
There are several answers to that question illustrated in the movie, but no definitive conclusion. A superhero can go on a secret mission that will actually end the entire globe spanning conflict in one stroke. A superhero can shame the leaders of the good country who are acting badly. A superhero can just do anything that helps, and be glad they made a difference in one life. A super hero can take the attention of enemies so a mass of soldiers can advance. A superhero can be a symbol to inspire others. Diana tries all of these.
WW also does an excellent of showing how Steven and Diana have different perspectives, and different ways of interpreting the same facts. Steve just sees a nihilistic mass of chaos, Diana sees a simple fight where the Bad Guy needs to be knocked down. And there's enough evidence to justify either of them as the movie goes on.
Diana of course, is fairly detached from reality and becomes a Don Quixote figure, increasingly forcing the facts of the world to fit her fantastic narrative (and to be fair to her, the world obliges by throwing up things like "the enemy general talks sinisterly about Greek gods, and is magically strong enough to give her a real fight.") This is what delusion looks like - she even picks up the paranoid parts, as she "realizes" that Ares has corrupted the good guys too. (She sounds exactly like someone faced with an intervention, who realizes all their friends are in on the conspiracy against them too.)
Her penultimate scene with Steven serves as a "rock bottom" traumatic encounter with the Real, where she has vanguished her imagined foe (the General-as-Ares) and realizes she was living a fantasy. Steven explains that humanity is just that bad on its own, and will never be saved because they deserve it but rather because we choose to save them (an extremely ethical and Christian point), and that Diana was deceiving herself, throwing her into utter despair.
At this point, Diana suffers a psychotic breakdown.
Her imaginations grabs the only other person she has talked with substantively since coming to the normal world that isn't on her elite team or someone she killed, and has him appear three hundred miles out of place. This vision taunts her, she punches at him and he disappears. No one else is around to see him. This is wholly "figment of your imagination" imagery.
For comparison, see the ground-breaking fight scene between Superman's good and evil instincts in Superman 3.
There's no plot explanation for how Superman was cloned and there are two of them, it's wholly representative of the battle going on in his mind. You better believe the DCU creators are familiar with this scene - not only is it a classic of the genre, but Snyder was heavily influenced by the 80's Superman movies. The Diana/Ares fight is extremely similar to this. No one else on the German air force base notices or cares about this other man blowing stuff up - because he's not really there. Diana is just going batshit, destroying everything. (Well most of the explosions are caused by the infiltrator's terrorism, but she interprets that as being caused by this Ares.)
It follows the typical rule of psychological fights. When something is shown that makes humanity look negative, Ares is strong. When something happens that reminds Diana of why she loves humanity, Diana is strong. Eventually the good side defeats the bad side, and then Diana gives a voiceover about how our light side and dark sides fight internally.
There's a lot more to be said about the movie, particularly Steven's Christian nihilism and Diana's active idealism and the characterization behind the secondary figures, that I'll enjoy re-watching the movie to pick up on more of. But this is at least a pretty good start for framing discussions of this entry in the DCU.
PS: It does need to be noted, that without Ares being a psychotic delusion, his appearance is totally incoherent. Diana has just killed a man she believes is responsible for the war, this is revealed as a tragic failure, and Steven gives an emotional and climactic speech about how "no one man is responsible for the war." The follow up to this is not "Oh, except this one bad guy. Yeah you have to kill him." His appearance is absurd, except as "continuing the fantasies Diana is dependent on."