Full disclosure, I have not read and have no intention to read, Midnight Sun. What a waste of time. Like watching a rerun of the Superbowl - same game, same score. It is just a rehash (different voice means very little) of a story for which we already know the ending.
The entire series ended with THE PERFECT set up for additional, new, story lines. The Cullen family has grown stronger and is already seen as a threat by the Vultari. The entire vampire world is poised on the edge of whether to believe the Vultari are the defacto law enforcement group, or just a bunch of self absorbed bullies. There is epic possibility for development of this conflict.
Secondly, Renesmee! Her powers are untested at best. As she grows, what else may develop, especially given her bloodline (pun intended).
Why waste time telling the same story over again except to just try to milk more money from it with out having to actually be creative!