The Borg, they used to be one of my favorite sci fi villains * sigh *
But like others here have pointed out, after their initial appears on "Q Who" and "Best of Both Worlds" which really established them as a major antagonist their concept started to suffer from both bad writing and the fact that you can not keep a villain invincible/unknowable permanently if the main characters should be able to win from them. (I doubt Star Trek would have ended with the Borg being victorious and some remnants of the Federation fleeing into deep space)
Personally I liked the assimilating part as I felt that it made them more frightening. They don't just take your civilization, technology and resources, they take you; your mind and your body and re-purpose it in a way they find useful. They can even revive the dead if they want to in order to make them into Borg drones.
I also did not dislike the team-up between Voyager's crew and the Borg during "Scorpion" even if it was obvious that the Borg would eventually betray the crew.
But the Borg Queen for me was one of the more damaging changes to the Borg (well Lore in Descent could also be considered part of that). I can understand that people who oversaw the production of First Contact wanted the villains to have a "face", a clear leader who leads "the bad guys" because facing off against an enemy doesn't show any reaction back than just sending in more troops and are more of a faceless force might work in an episode of a book but doesn't always make an exciting storyline. Even in natural disaster movies there are still clearly human villains. (the person who would discard the lives of others to save themselves, the foolish politician who underestimated the threat etc)
I saw her as a "voice of the Collective" as well at first, being a representation of the Collective rather than some kind of leader who controls the hive mind, but that changed in episodes like "Dark Frontier" and "Unimatrix Zero" in which she became the ruler of the Collective, now the swarm and displayed emotions like obsession and anger.
The Borg recovered a little in Enterprise Season 2 "Regeneration" but they still ended up defeated because they were written as weaker (and a bit foolish) and not because the main crew managed come with a clever strategy or other insight that helped them determine a weakness. Phlox could perhaps have been used for this.
So the Borg could never have lasted for a long time but I wish their final appearances on the screen weren't so poor.
My point is I don't believe the villains (Insurrection, & Nemesis) after First Contact were in the league of the Borg.
Yeah, the So'na and Shinzon and the Remans were never in the same league as the Borg. Neither had a good prior build up. (the So'na really should have appeared in DSN)
So if you had a chance to write or develop a Borg movie after FIRST CONTACT, what would've been your direction for Star Trek's space zombies? Or what were the potential of the Borg you saw where the production should've explored and why?
If the budget was unlimited I might have gone for something like other posters here mentioned; a big epic with major stakes as the Borg invade the Federation and neighboring civilizations in large numbers with the intention of assimilating everything.
And while Starfleet and the Federation have to deal with the invasion itself they also have to deal with displaced refugees, species and civilizations hostile towards the Federation seeing opportunities to expand their own territory or just in and steal whatever they can, and the realization that the Federation might actually fall which also puts its leadership under strain.
What I don't have yet is a solution for the crisis but I would not want it to rely on a deus ex machina. It would have to be something that has been introduced before.
The Borg Resistance and the Omega Molecule could perhaps be brought in but that would require the movie watcher to know Voyager and having seen the episodes in which these appeared.
What also would be cool if the major civilizations of the Alpha and Beta Quadrant band together to face this threat so that various recognizable faces could appear in the movie.
In the end the Collective would be defeated or weakened enough that it no longer poses a major threat and explained why they would not re appear in follow up series and movies.