I get what you want to say, but there are ways they can putt all these guys as cameos. From top of my had, Cyborg could be talented guy Batman asks for help and then some explosion happens(latter they reveal they managed to keep him alive as Cyborg. Aquaman could just walk out from the see and thats all and so on...And I'm sure they thought about this more than 2 minutes .
Given what's been done in some other films I have a feeling that half of the film will be devoted to the 'origin stories' of the characters with only 45min-1hr devoted to the actual "plot".
IMO if they were going to include characters like Wonder Woman or Cyborg in the franchise they should've made the introductory films for these characters before creating a "Justice League" type film (and this should have been a separate film from Batman vs. Superman; just like the Iron Man and Thor films don't include cameos from the other Avengers other than quick text references).
Like the other guy said I think DC's trying to 'catch up to Marvel' by cramming all of this into 1 film; maybe they'll play it smart and only include quick references to the other heroes instead of extensive backstory but I have my doubts since the superhero film industry doesn't try too hard to "be different" and usually sticks to a predictable formula.