On the Death Star thing, its not called "Death Star 3", but at the end of the day its a giant Imperial superweapon where the climax takes place. Frankly, I'm surprised people didn't think the movie would end with something like that. They're trying to reintroduce Star Wars to the general public, how else would you end your new standalone movie but with a big war in the stars? What else could give your new Star Wars film the sense of finality and closure, and have such a simple concept that any child in the audience can grasp("Giant bad guy weapon, Rebels vs Empire, got it")? People can point to Empire Strikes Back not having one, but its a different context. As a sequel/middle chapter, Empire had the benefit of not having to start small building up its characters; it could assume our empathy and have its biggest battle right in the beginning(same thing with Revenge of the Sith). Also, since it knows you're gonna see the next one anyway, it can end with a non-ending, with our heroes not accomplishing a damn thing and there's no sense of finality whatsoever.
The Force Awakens can't do that. It can build on the past and it can point to the future, but it needs to be a satisfying movie in its own right, with a beginning middle and end, with character arcs that aren't closed but are in a very different place then they were in the beginning, with an ending that provides a cathartic sense of closure but also points the way to further adventures.