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.
Eh, no reason for Q, TOS had plenty of their own "Q"-like adversaries to choose from that could be used.
They could use Charlie X and have Spock and Uhura singing and playing a tune together (Since that was one of their flirtiest moments in TOS), or as SeerSGB says Trelane, or the Witches from the Halloween episode...
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?