I wish that INS had been a Dominion War movie. As others have pointed out, doing something like the Battle of Betazed could've been a side story that had personal consequences for the TNG cast, in addition to perhaps bringing back Majel Barrett. As for the casual fans not knowing what the Dominion War is, that could've been done with a quick overview at the beginning of the film, or an action sequence or some dialogue. I mean, the Clone Wars cartoon does it every week in about a minute or so. Further, I was just checking out the opening of the DS9 pilot and it had some text at the beginning. This could've been done for a Dominion War film.
I thought the Son'a weren't bad villains at all, with cool ships, and a nice super weapon, but it would've been great to see the Jem'Hadar up on the big screen. F. Murray Abraham's character could've worked just as easily as a Vorta or Cardassian. Heck they might have even had a reason to bring back Madred to also make it personal again for Picard. Also some big battles with Klingon, Romulan, Cardassian, Starfleet, and Dominion ships. I thought it was a missed opportunity.
Further, if that INS had been a hit it might have funneled new fans into watching the rest of the story on DS9, upping their ratings.