I still want the Borg. New threat for Kirk. Fan favorite. With the popularity of Zombies right now, if the Borg were made scary, it could be fun. And they'd be a villain that could be hyped up.
My runner up would be the Talosians.
I don't see a lighthearted sequel though even if Tribbles or Trelene or Gangsters making the cut. So, no Voyage Home.
Give me Borg or Talosians. Or what if the Borg tried to assimilate the Talosians?