Here's the first and arguably the most well-known story thread: Star Trek II: The Wrath of Khan. In the second season episode "Space Seed", The Enterprise comes across a derelict space ship identified as an ancient Earth vessel designed to carry its crew in cryogenic sleep for interstellar travel, carrying would-be world conqueror, Khan Noonian Singh. Star Trek II: TWOK sees Khan now living on a desert planet, devoid of life and managing to survive in the harshest of conditions. He and his remaining followers are encountered by mistake by the crew of the USS Reliant. You see the planet next to the lush one that Khan and his followers were living on exploded, forcing their planet to shift orbits and change its climatological conditions. Khan now has a starship of his own and vows to hunt Kirk down and exact his revenge. It's a perfect example of what I'm talking about. With Ricardo Montalban reprising his role as Khan arguably his most well-known role, Montalban brought the same passionate fire of a thwarted conqueror that he had in the original 1967 role to the 1982 aged, but vengeance-filled Khan. It's a "Twenty Years Later" angle and an incredibly successful example of what Trek writers today should be doing: mining the past episodes to exploit great material for future stories that continue to entertain us and continue the legacy rather than wasting time in trying to re-invent the wheel. TWOK shows us that if the wheel ain't broken don't fix it. Then, there are the novels.