One thing I wouldn't do is tie Insurrection in with the Dominion War, or any of the films in with the continuity of whatever TV series was running at the time (this has been suggested more than a few times over the years). The film has to stand on its own to appeal to a broader audience, not only at the time of release but also in its afterlife. The films are not slotted into the syndication reruns of any series. Insurrection could be played anywhere at any time. A 10 pm showing on HBO would reach more casual viewers who might stop at the movie and check it out. By tying the films into the TV series too closely, one takes a chance of losing that audience, who has to be brought up to speed. It would be like running a random episode without any of the others around it. If it is a standalone episode, no problem. If it's a vital cog in the ongoing arc, then it's not as effective.
Sure, a few lines of dialog could probably do it, but the farther away in time we get from a discontinued TV show, the less relevant it becomes. Tying the film directly to it would make it less important because it's slaved to another show rather than the series the film actually is a sequel of. If you're going to tie a TNG movie to any series, it should be TNG itself. But I always preferred the movies to work on their own, so one wouldn't need to follow the series to get it.
I never understand this argument. There will hardly be anyone out there who doesn't know that Star Trek is a major franchise (and had been for some 30 years at that point), with many tv series and films already out there. So you would expect there to be a larger universe out there in which this movie takes place (on a side note, that's one of the reasons watching First Contact was a catalyst for me to start watching the tv shows). I agree that the movie shouldn't just be another installment in a larger arc, but why couldn't the general backdrop of the Trekverse at that point be used? Why not make the Dominion war an extra factor in the necessity to get whatever medical mcguffin they were looking for? After all, they do already mention it, iirc. Surely audiences can handle a bit of background.
Keep things clear and simple. Sona are bad guys, starfleet good guys.
For some movies that might be a good idea, but why for Insurrection? The title itself implies there is some Starfleet vs. Starfleet about to happen. If the moral dilemma at the heart of the story would've been presented as a real dilemma, where there are valid pros and cons on both sides of the story, without one side turning into a comic book villain, then there would have been enough justification to have different Starfleet officers on different sides of the question. And if the argument is that audiences can't handle this kind of 'complexity', that is very denigrating. Similar story lines, in which friends are pitted against friends for ideological reasons, pop up in other genres as well, why would scifi audiences not be up for it?
It's been a long long time since I saw INS, but I'll try to put out a few points of potential improvement:
-Do more with the backdrop of the Dominion war. It doesn't need to be on the foreground, but it needs to play a role. Just like any movie set during WW2 doesn't necessarily need to deal with battles and whatnot, but at least the war needs to be acknowledged as playing a major role in everyday life. For example, show wounded coming into a Federation hospital, showing the need for special whatchamacallit they were looking for on the planet. Perhaps even have one of the main characters be related to one of the wounded, to up the ante, and make his or her choice against the Baku even more believable. (Edited to add: Or possibly, explicitly, situate the movie before or after the war. But I don't see why you would throw away such a great ingredient to add drama and necessity, if you get it handed to you for free.)
-Speaking of main characters: I want to see the TNG characters in a TNG film, not a whole slew of guest stars. Son'a, Baku, I don't know who... More crew interaction please. Having them, at least initially, at odds about which course of action to take, is the perfect excuse for this.
-Make the universe bigger. Related to the first point, but show us more of the consequences of the actions on the Baku planet. Show the Federation council deliberating about what to do about it. Anything like this. If you want, you can even get partners-in-the-war Klingons and Romulans chiming in, because they will be affected to.
-Lose the Son'a, or do something better with them than making them comic book villains. There is an interesting moral dilemma at the heart of the story, which could lead to interesting conflicts, without the obligatory villain in there. Perhaps that doesn't jive with the Star Trek philosophy (Starfleet vs. Starfleet), but in that case, why try to make a movie about an Insurrection in the first place? And if the argument is that the movie going audience expects a villain in their movies, I'm tempted to say again: give it (us!) a bit more credit.
-Consequences! Have Picard (and eventually possibly the rest of the crew) really take a stand against Starfleet, if he feels their actions go against his morality. Have him take a stand. Have him lose his command. End the movie with Picard leaving Starfleet. What a wonderful set up for Nemesis: Picard, disillusioned about Starfleet to which he has given his whole life up until now. And there is Shinzon, tempting him away even further. Praying upon his uncertainties. Give us the feeling that something important (not forgettable aliens of the week) is in the balance: Picard, this man we've followed through 7 years of tv series and 3 or 4 movies. Is he going to fall to despair, or will he find something new to fight for? Something to restore our confidence in the Federation, before the TNG crew takes a bow and leaves us behind.