Please, no Borg. The Borg belong to TNG, first. There's no reason why they can't appear in the TOS world, but out of universe, they're TNG just as Q is.
They also stopped being novel when they had a queen. They went from mindless, relentless, unreasoning, unable to reason with beings who behaved like a plague of locusts to just another baddie led by an evil being. So what's new about that?
The other thing is this would be basically a war movie and more or less a remake of FC (in at least the same vein as those who think STID is just TWOK revisited).
One last point, after the anti-militarization-let-get-back-to-what-Starfleet's-primary-role-is-theme of STID, no big scale shoot-em up wars in III for me, please. Out of universe, it's a little cynical to have a story that says Starfleet is about exploration, so let's end with Kirk reminding folks of that as he starts his five year mission, but in reality, war sells, so -- . I think some kind of good action story can come from another direction and be just as whiz-bang. It only takes imagination.