While I love the idea of the TNG actually assisting in the Dominion War, the idea of a movie exploring that is just not feasible. ANd here's why:
Making the a TNG movie focused on Dominion War would've been bad on at least three levels:
1. The Dominion War was DS9's main focus, centering on it in a TNG movie would've impacted DS9's storyline.
2. Being a DS9 focus what was going on may not be knowledgeable to Trek fans familiar with TNG but not DS9.
3. The Dominion War certainly wouldn't be knowledgeable to even more casual fan or general audiences.
But, in short, the first point is the biggest one. The Dominion War was DS9's baby, no reason for TNG to do anything with it.
In short, Trekker4747 got the point, and I agree. The Dominion War is, in fact, TOO closely associated with DS9, that it would ultimately feel like a deviation from it, and ultimately, it'd only be handled terribly. I mean, Berman!
That said, all things considered... The film should've been set either before or after the war. Setting INS during it was a bad, bad mistake, because if nothing else, it made Picard and co. extremely stupid by opposing the one chance the Federation had of a regenerative facility for their soldiers during the damn war in which so many of them died, even after the Romulans joining them. Oh, and calling them traitors because of that wouldn't be too far off.
So after that first step, it all depends. But part of the reason the film sucked was because of Berman and Stewart's involvement. I hate to say it, but both of them should've trusted the late Michael Piller more. His original idea was really interesting, and with a few tweaks, it'd be a great follow-up to FC.