Even with the heroes known you need to write a movie for a “clean” audience, one that doesn’t know the characters before hand. Otherwise you get into the problem with X-Men 3 where they had somebody show up for 30 seconds to use their powers then never be heard from again, sure the hardcore X-Men fans recognized the characters when the power got used and could comprehend their place in the story but for the rest of the audience it was a serious case of “who’s that, what can they do, where’d the go”. Then you have another problem, with so many of the characters involved in movie franchises already you’d need to figure out how Secret Wars fits in the other movie series, will the Spidey in the Secret Wars movies be the same Spidey that’s in the Spiderman movies, if yes will the next Spiderman movie be pre or post Secret Wars, will any subsequent Spiderman movie be post Secret Wars; and the same set of questions need to be dealt with for all the characters heroes and villains alike. Movie companies have no practice managing things like that.
but for the rest of the audience it was a serious case of “who’s that, what can they do, where’d the go”.
No doubt that was true for a portion of the audience, but I would hazard a guess that the true success of the films was decided by said hardcore fans. It's people my (our?) age in their early to mid 30's that are making these films successful because it was us who grew up on this stuff.