First of all, I think it goes without saying that the following post will include POSSIBLE spoilers.
With that said, I will tackle the most important thing that we learnt from the trailer, and that's on everybody's mind: Doomsday.
Showing Doomsday: Why?
Yes, they did show him. People argue that his appearance was not only underwhelming because of his design, and deconstructing the whole tension built on the Batman vs Superman rivalry, but also that he should have been kept as a surprise. Well, it was almost impossible to be kept as a "surprise":
- The official synopsis mentioned that a threat will emerge, which, evidently will unite them both.
- This, and Man of Steel are foundation of DCEU, it's not called "Dawn of Justice" for nothing, the Justice League begins to be forged with this movie - and I think this is a reason to celebrate not to bash.
- Understandable for the general audience!
Going after the general audience!
This will most probably be the cinema trailer, where a larger audience will see it and decide if it will see it or not. They, of course, know who Batman and Superman are, and they're basic background, but they are not aware of the DCEU that is being constructed. Of course they will ask themselves: "How can Batman fight Superman? This is so stupid", but showing that a larger threat will emerge and also throw in other elements into the mixture, it will become much more palpable.
I can say this from my own experience, I have friends who were confused about the sustainability of the subject of Batman confronting Superman, even if you throw "Kryptonite" element into the equation. But after seeing this trailer, it didn't got just more enjoyable but more logical (not everyone read Frank Millers TDKR), so it was a good audition for the larger audience, which, by the way, sells most of the tickets. The Dark Knight record sales (at that time) was not made possible just by Batman fans but from regular cinema goers.
Why Doomsday doesn't look bad!
I personally do not think that showing Doomsday was an easy decision for Snyder to make, however, as we all know, the character evolves. So it's most probably just the initial version of Doomsday, for sure, one that got out of control - maybe - before Luthor Jr. finished the whole process.
This is NOT the ending! (more possible debated spoilers ahead)
Lex Luthor is the ultimate villain of the movie, and there are action figures now being manufactured that showed him in the iconic mechanized suit - of course, you cannot count 100% of merchandise, but I consider it a worth mentioning input to our analysis.
Batman, Superman do pause their fight, there is no conclusion that it will end right after possibly Doomsday is defeated... And
It's hard to not mention that Doomsday killed Superman, and since Snyder honors the source material (most of the time), I'm sure he took this into consideration.
Superman could be killed (that's a plot twist over there), and of course resurrected for Justice League through different methods, like extreme sunlight exposure.
What if Doomsday and Luthor are not the end game, but and Alien force, as the ambiguous and mysterious desert scene implies, regarding those winged creatures fighting soldiers. (Apokalips tease?) - nightmare, premonition or actual reality?
In the end, the trailer was much more opened that we ever expected, it did serve sell purposes and marketing interests to attract and be more palpable to a general audience. Also, Doomsday was hardly a spoiler as it was a confirmation to our guesses that complimented the already revealing synopsis. On the same idea, it's wrong to guess that this is the hi's final look.
All of the trailers were very careful to exploit only some parts of the movie, key parts (of course), but it's absurd to say that they've revealed the whole movie, other than what was already announced in the synopsis.