The synopsis tells us mostly what we already knew. But there is a potential interesting point in the synopsis."Putting mankind in greater danger than it's ever known before." Just so you know they aren't talking about Lex Luthor (well they shouldn't be), because Lex Luthor ISN'T a threat to MANKIND. I'm sure you know that Lex, in his own way, is saving the world. So its not him. If it is, then they've fucked it up.
So that begs the question: "Who is it?" Well that would be a very good question. It says a danger that is more dangerous than ever before. The DCCU may be very small (with a total of one movie), but still Zod was a HUGE threat to mankind. He was a KRYPTONIAN. So who or what is stronger and more dangerous than a kryptonian? LOADS of things and people! But we can exclude people like Darkseid (or maybe not) because it says the threat appears quickly and Darkseid wouldn't do that. He sends his parademons first and he is very tactical. But more importantly, Darkseid would overwhelm the movie. They would save Darkseid for the Justice League movie.
So it has to be: already on Earth and more dangerous than Superman. Now there have been rumours that, even though Lex Luthor is the main antagonist, there is a secondary villain. The brawn to the brain. Of course I am talking about DOOMSDAY! He would be perfect! They could have him in a solo Superman movie and in the comics he was killed by Superman and Superman alone. But in BvS, Superman has the help of Batman and Wonder Woman! Of course this is at the rumour level but still the rumours refuse to die. The rumour is that Doomsday is created from Zod's corpse, which sounds shit and is a heavy insult to hardcore comic book fans. But then again one of the two screen writers is Davis Goyer who has shown lack of respect to comic books and his own audience when he HEAVILY insulted the Martian Manhunter. He also showed comic book ignorance. So yeah, you can look that up. Also, there are weaker rumours that Lex will don his iconic warsuit and Metallo will make an appearance. So [Batman v Superman: Dawn of Justice](tag:711870) has already got a shitton of characters, but can they juggle all those character? I guess we'll just have to wait and see.
Well, I hope you found that insightful.
Batman v Superman: Dawn of Justice will be released on March 25th 2016.