Year of Hell
In Star Trek: Voyager's fourth season, there exists a two-parter early on called "The Year of Hell". In it, the Voyager runs afoul with a Krenim leader named Annorax. Throughout the two-parter, the two sides exchange salvos in a battle that lasts, as you guessed it, a year. It's not a consistent fight, with many weeks and months in between dust-ups but Annorax cannot only batter Voyager but change the timeline that Voyager is in, worsening the situation on the ship.
Due to that, the Voyager is barely running as the second episode comes to a close. In fact, with the ship on its last legs and most (if not the entire crew dead at this point) Captain Kathryn Janeway rams the Voyager into Annorax's ship, killing everyone involved. The resulting explosion resets everything to a point in time before Annorax's involvement in the timeline. Resulting in Voyager being back to where they were at the start of the two-parter.
What if that wasn't the case, however? What if, with some changes, the timeline didn't get reset? Voyager and most of its core crew could remain intact, but you could easily start telling a story in season five of Janeway looking for new recruits for the ship. Characters like Harry Kim could finally be promoted and maybe, just maybe, the entire year of hell could finally connect Janeway and Chakotay in a way all fans have wanted for years.
Instead, that entire series of events never technically happened. Narratively, it's a real shame to lose out on that much potential character development and growth.