I thought INS was decent and I liked the Son'a, especially their ships. Though I felt that they were under developed, and wished they had showed up on DS9. But if I had my druthers I liked the original idea of the Romulans being involved. A potential situation that could drive a wedge between the Federation and Romulans in the midst of the Dominion War would've had much bigger stakes than even those involved in INS (though those stakes were very important too). Seeing the Jem'Hadar on the big screen would've been awesome too.
Also, I wouldn't have made the Ba'ku lily white. And I would've had a rejuvenated Picard romance Crusher instead of bringing in a one-movie romance. Lastly, the effects of the Ba'ku planet would permanently restore Geordi's sight. If for no other reason to give LeVar Burton a break from having to put in contacts after years of wearing that visor.