BUT this pretty much answers most of the questions asked here. :D
'Some fans have asked if the upcoming Steins;Gate film, whether it was a cut-paste the anime or have original material. Chiyomaru has revealed that the film Steins;Gate will have a new story, different from the anime. He has not made clear if it will cover any of the other branches of the visual novel unexplored in the anime or not (although the end is the same, there are several alternative routes from other world lines) but it will be something new. Among the things he noted was that it will not be the story of future Suzuha. (Which is told in her manga series Boukan no Rebellion)'
Considering how quickly the choice of making a movie came about, it obviously must have been thought about and decided well before the series was even over. Because of this, they knew exactly what they wanted to make the movie out of, and this leads me to believe that it may be one of their Steins;Gate items that weren't so elaborated on. What'd that be? The Gamma Worldline: in a time well about the 2% threshold, Okarin finds himself to be a rounder, working with Moeka and Braun to sabotage the FGL.