Personally speaking, I always kinda knew that Twicorn was coming. No, seriously, I foresaw it as far back as the Pilot two-parter. I did so by deducing the nature of the Celestia, Twilight, and Luna's names. Celestia, is based on "Celestial", which is applied to bright, majestic, and shining things like the sun. Luna has always been a word strongly related to the moon and nighttime. Well, Twilight is one of two midway points between the two: the dawn and twilight hours, of course.
In my mind, it always made perfect sense for Twilight to become an alicorn. Okay, sure, my originalprediction had Celestia or Luna stepping down and letting a new generation of princess handle the sun or moon, which didn't happen, but hey, go me for guessing that Twicorn would happen.
Yeah, bronies take fanfics WAY too serioously. It's like they forgot just what the term "fanfic" means after a while.