Civil War was a superhero vs. superhero movie and they end up sort of splitted. Of course, the finale implies the possibility of a future union, but it remains to be seen in a future film. It's in the closure itself, not in the beginning of the final act.
Not quite the same. I'm talking about the cliche where two superheroes meet for the first time, fight over some silly misunderstanding or manipulation by the villain, and end up as friends. BvS was just an overblown and exceptionally stupid rehash of that hoary trope.
Civil War was a very different story, about heroes who knew each other well coming into conflict over something of genuine substance. (There was some manipulation by a villain involved, but only to cultivate and exacerbate the genuine sources of conflict.)
Still, most trailers do give up much of the movies plot, particullary blockbusters.
There's nothing new about that. Heck, '70s and '80s TV shows would routinely
begin an episode with a preview of upcoming scenes, covering pretty much the whole plot, usually including scenes from the climax. They were called "teasers" for a reason -- giving you a glimpse of the story and the action so you'd be curious to see how the whole thing played out.
The value of a story is not in knowing
what happens, it's in knowing
how it happens. Knowing the plot structure in advance doesn't ruin the experience, because the experience is not just about the sterile facts, it's about the execution, the performances, the dialogue, the details. Yes, in
some cases, knowing a certain fact in advance can damage the surprise, but that's only for certain types of thing in certain types of story.