How I see it.
Let me first say. That a movie like this, is nothing new. It has been done tons of times before. With good and bad results.
I think this is just a way of giving the fans more to look forward to. While in the meantime being busy with rethinking and making a 4th season. And I agree with the filling a gap part. It is definately too fil in a gap. Or at least show more hidden/possibile scenes that occured during that time. Even though they use the words of "not a true universe". Though there are other possibilities but these two are the most likely.
However since it's only around 2hours they have to pack it fast.
First of all. It's going to be based around the relationship of Nanoha and Fate. This will not be about fighting, alicia, suzuka, ferretboy, whatever. No it will be Nanoha and Fate centered. That's for sure. No they are not going to give Yuuno anymore change.
http://kurogane.animeblogger.net/ima...uncment_01.jpg
Why for the simple reason that Nanoha and Fate are a couple. They are pictured together from season 1 and even in the SS there is clear signs of this. Yes it's true(for all you Yuuno fans). 7 Arcs has made the seasons in such a way to grow closer to Nanoha and Fate and match them together. The didn't do this by mistake or anything. In fact no the realised that Nanoha and Fate where populair, selling good and are just a great couple. With Fate-Mama in StrikerS they absolutely triggered the lez-flag. And I am happy they aren't affraid to show/hint on lesbian relationship in an anime. I won't go into detail with the seiyuu but like Preston said both Seiyuus have now in one way or another confirmed NxF are a couple. And I agreed with alot of posts from Keroko as well.
So without any doubt the movie is an (alternate universe) of season 1. So it doesn't belong to the time lime. That's a little bit what is saddening me. I would have loved it to be genuine in a way. But it seems the wanted to upgrade the battle effects and devices as well, while they where going to show hidden scenes anyway.
Anyway this movie is going to be more Nanoha and Fate fluff. And I think in the end there may be a major climax in it. They might end in holding hands walking away(oh wait they allready did that
). Now we (as Yuri fans) all would like to know are they going to kiss in the end. You know the bridge scene and all(most probably it will happen there). I think the chance is high for this. Even considering there a populair yuri vid on nico(no not youtube). And considering they upped the Yuri scenes with every season.