They want to promote this movie in the most effective way so it's understandable that they would want to release images off them to get the audience amped up. It has been said that Aquaman's will have a tiny cameo. Wonder Woman will also have a small role nothing major. They are setting this up for a Justice League movie that comes out the year after Batman v Superman so that's why these heroes will have appear in small roles. I bet Aquaman's and Wonder Woman aren't the only heroes, besides Superman and Batman, that you'll see in the movie.
The first Avengers movie had like 7 or 8 main characters, the second movie will have 12, the first critically acclaimed x-men movie was made out of an ensemble of characters, the critically acclaimed Watchmen movie had and ensamble of characters. You can have as many characters as you want just as long as it fits in with the script and they serve an actual purpose.
The wrong way to do it is like how they did it with Spider-Man 3 and The Amazing Spider-Man 2, IMO of course.
