Luna won't become Nightmare moon again unless some evil force makes her do it. Besides, Luna's original reason for being banished was likely because she was going against Celestia, perhaps because their elements of harmony were mixed up. This particular headcanon author has a lot of videos that make sense in that regard: http://www.youtube.com/watch?v=RWwpP4zPHFU
So, I recently came across this brony's youtube page. I think it's some pretty good stuff. The first video is ok, but I really like their other submissions. If it's true, it makes a whole lot of sense! THe pony writers would be brilliant (more so than they already are )
http://www.youtube.com/watch?v=DG9ZyqYu1Mg