I strongly believe that the books' concepts would be used in the movie (if they ever make one) because they were part of the ideas to be used in season three (if there had been one).
I do think something dramatic will be done because it has to propel the characters forward, but it doesn't mean that Joshua would have to die. The thing with the movie, they would probably focus it on only a few characters opposed to a huge group of them so it'd most like focus on Max, Logan, and Alec. Let's face it, those three have become the face to Dark Angel and most fans like all three. I like Joshua loads too and I'd love to see his friendship with Max remain intact. Also, what about all Max's human friends? I want to see OC, Sketch, Herbal, Kendra...all them in it.
The storyline, I think will be set years after where they left the show off because it has been a large gap in time since then. I hope the movie could tie up loose ends on Lydecker, Sandeman, White's Breeding cult, and the virus. I know most of it is cleared in the books so that is where I think the books might be used in the script.
Who knows really other than Cameron? I hope he plans something good for the fans and I just wouldn't want to be disappointed. And it wouldn't work if all the actors weren't on board to participate. I know Jessica says she is so I wonder about Micheal, Jensen...they both work on television series which makes it hard to just go work on a film. Ergh...enough of my babble, I'm not saying anything that everone hasn't all ready suggested.