10 Star Trek Episodes That Paid Off WAY Later
4. Space Seed
Ok, no, this isn't just going to talk about Star Trek II: The Wrath of Khan, although that did come sixteen years after the release of Ricardo Montalban's first appearance in Star Trek. While Khan will generally still consistently be referred to as the greatest villain in the franchise, it is the Eugenics Wars that are introduced in this episode that takes point.
The rise of the Supermen and the wars to defeat them would return many times throughout Trek history. In Deep Space Nine, Dr. Bashir is revealed to have undergone genetic resequencing as a child, which leads to a tribunal, and an evocation of Khan's name. Though Bashir is spared the legal ramifications, he later helps to treat others who are like him, yet not so fortunate.
Star Trek: Enterprise would go further. In the three-part story centering on the Augments and Arik Soong, genetic resequencing is again brought into play. Here though, a group of these augmented humans goes on a rampage, proving to be at least equal to Starfleet's best along the way. This augmentation is acquired by the Klingons, leading to a full circle moment in Star Trek.
The augmentation virus that infects the entire Klingon race erases their head ridges, leading to the smooth-headed Klingons best remembered from The Original Series. Though it took forty years to get there, Khan and the events of Space Seed end up retroactively explaining the change in the Klingons overall.