A non-Borg Threatened 24th Century would be interesting.
How about a 'Q Who' type episode with Kirk and crew - They meet up with Q who propells them into the Delta Quadrant to meet the Borg a la Picard style. But instead of getting a bloody nose and running like Picard, Kirk and crew kicks their collective asses, freeing them all, much to Q's dislike.
Fanwanky, I know.
Change Q to Trelane.
Please don't. Trelane was one of the worst characters TOS ever came up with. Literally a semi-ominiscient baby, who spent the entire episode whining and complaining. It was decent enough as a comedic episode of a tv series, but it would be complete crap as a film.
Interesting - I've never thought of "The Squire of Gothos" as an especially comedic episode, and it was exactly the notion that Trelane was a child which made it work for me.
"Omniscient" or "semi-omniscient" might not be quite the right descriptors but if you imagine a character who's the rough equivalent of a seven-year-old, with all the capacity for simple joy and sudden anger and meanness and casual cruelty which that entails, and you allow him the ability (within the defined limits of his "play area") to do anything he wants and manipulate/toy with/torment anything he chooses - well, that's potentially a pretty frightening sort of person for our crew to encounter. I thought that William Campbell did a very effective job at conveying the range of attitudes and behaviors called for by the story.
All that said, I'd just as soon see a new adversary in the next outing than I would a re-imagined Trelane, a spiffed-up and scarier Borg, or a V'Ger-Nomad-Doomsday-whale probe sort of thingy. I could even do without the Klingons, though at the moment that seems less likely.