You have to give them time. It's only been four years, after all, since Iron Man came out. And while they've focused on their biggest characters, each character has only received one movie so far, save for Iron Man. I don't think we need to worry about them only focusing on their biggest characters, though. After all, they've already seriously talked about doing an Ant-Man and Dr. Strange movies, crowed about having the rights to the Punisher and Blade back, and have even mentioned the Guardians of the Galaxy, of all people. So I doubt we have anything to worry about, as they've already doubled WB's character output in only four years while Warner Brothers has over 20 years to put some serious work into characters who aren't Batman and Superman, and completely futz it up when they try to bring in a third.