Perhaps a bit easier than yesterday but still one that might present a few possibilities.
True indeed because I started off with three - one fairly obvious, one along similar lines and then I finally settled on my third.
Just to clarify this can be something you saw and wanted to see again, an invention that dropped into the mists of never, a story that just went, well, nowhere.
I'll drop the ones I thought of into the comments later but my final choice is...the telekinetic powers possessed by the Vorta.
The what? Yes, the powers displayed by Eris (Molly Hagan) in The Jem'Hadar and then never seen, spoken or even whispered again. Now it could be that they were a technological addition produced purely for that encounter in the Deep Space Nine season two closer but it's just a little bit of a head-scratcher what the reason for dropping them might have been. Eris was supposed to have appeared in The Search (21 years old in September) but didn't so we might have found out if she was the only Vorta with the powers/genetic defect/something else.
Most likely it was a little too powerful a plot device to give to the Vorta and was immediately forgotten as they developed the servants of the Founders for the third year and beyond. Having them stronger than the Founders might not have been wise!
So what didn't Star Trek finish off? What didn't it explore after that initial encounter?