Sort of cousins... the void surrounding the giant space amoeba and Nagilum's hole in space. ("THE IMMUNITY SYNDROME" and "Where No Silence Lease")
I will very rarely speak ill of Data... but he should have at least called it 'somewhere' in the realm of what they encountered before Nagilum gobbled them.
The Enterprise has to get a one of a kind medicine, and a member of the crew basically has to fight to get it. Am I talking about "THE CLOUD MINDERS" or "Code of Honor"? The answer is both.
A prearranged marriage where the groom/bride is on another world, but they leave the one on the Enterprise for someone else. I'm talking about "AMOK TIME" and "Haven".
"THE DEADLY YEARS" and "Unnatural Selection" both find the Enterprise discovering dead people who died of extreme old age before their time, and a member or members of their crew are infected. (Though this plot has been used in many shows.)
That's the first two seasons I can think of... at least, where it's more than just surface similarity. (Pike's chair and Jameson's chair, two Kirks and Data/Lore, etc.)
There might be more, but I'll have to think on it further.