Natasha Yar's return
The decision to kill off Natasha Yar in Star Trek: The Next Generation came down to issues that Yar's actress, Denise Crosby had with the people running the show at the time. Likewise, Gates McFadden had similar issues, which resulted in her joining Crosby and leaving the show at the end of the first season. McFadden's character, Dr. Beverely Crusher, was not killed off, however.
By season three, things had stabilized on the show and those more toxic voices behind the scenes were gone, opening the door for Crosby to return as Yar once again. However, this time as a slightly different version of the character. In another alternate timeline story, the Enterprise-D existed in an era where the Federation was more militaristic than they are seen as. That's because the Enterprise-C never met the end history told us about, resulting in a timeline shift. This brought Yar back.
In the storyline, the Enterprise-D would find the Enterprise-C in their timeline, and help it return to theirs. Even though everyone involved knew the Enterprirse-C would be destroyed. Yar, sensing she didn't belong would go back with the Enterprise-C, seemingly dying with the rest of the crew. She didn't and would later give birth to the half-Romulan, half-Human villain, Sela.
If Crosby's time on the show had been different, Yar could've come back in a similar way as she originally did in season three "Yesterday's Enterprise" but this time stuck with her. Allowing her to find her way back into her role on the ship, having lost over a year of time and trying to figure out how she fit again with the crew.
Instead, what we got wasn't bad. Sela was a great villain, but Yar was a great character, one unexplored and left with lots of potential on the table for storytelling purposes.