Star Trek 4: 8 Villains We Might See
3. The Borg
Perhaps the most popular enemies of the Federation besides the Klingons, the Borg offer the most potential of any classic villain, and if done correctly could create a truly unique fourth film.
It is hard to respond to the Borg hivemind with anything but violence. They are less of an antagonist, and more of a force of nature, driven only to assimilate. In some ways they are more terrifying than Doctor Who's Cybermen. When the Enterprise comes across the Borg Cube, their only option is fight to survive.
Going up against the Borg would raise the stakes for the team- hopefully it would lead to at least one meaningful death at the hands of the Borg. Not even Khan could kill Kirk- there's every chance, for example, Uhura or Sulu could be assimilated and lost forever. The Enterprise family torn apart by an enemy that can't be reasoned with, or negotiated with.
Jonathan Frakes did a fantastic job directing a distinctive and stylised Borg movie with 1996's First Contact. Bringing Frakes on board would not only give the next movie a bit of old-school magic, but Frakes' experience in directing the Borg would ensure an enemy with real presence.