I'd through in a line to the effect of, "That's what the California-class ships are for!"
I know the backstory that the slugs were the start of the idea for the Borg, but I don't see why they couldn't have done something with them even after the Borg appeared. While they both take over someone, the Conspiracy aliens might just be more terrifying, since you at least see the Borg coming. Those slugs took over the admiralty with relative ease. After their loss in "Conspiracy," they could have found another way to infiltrate and this time not arouse suspicions.
I do think it's a testament to the writing staff (or maybe a condemnation of our creativity) that the majority of these suggestions are follow-ups. TNG did a great job of giving us a variety of episodes over the 7 years. They dabbled in different genres (You've got your horror episodes, your love interest episodes, your comedy episodes, a little Noir/private detective, fantasy episodes, etc.).
I think, really, the only thing missing is a musical episode. The crew could have encountered a species that communicates only through music and song. Or Q could force them to sing instead of talk. With the latter, imagine a tense situation with the Romulans, where Picard can only sing to communicate with them in an attempt to diffuse the situation.
On the other hand, maybe it's for the best we never got a musical episode!