u/Mr_Muda_Himself_V3

Luke/Leia (Star Wars)- George Lucas originally planned for it to be revealed that Luke had a sister In Return Of The Jedi, however during the writing process he realize that introducing yet another secret family member to the story in the final installment probably wouldn’t go over well, so he retooled the idea into making Leia Luke’s sister, which abruptly killed the love triangle they were doing with Him,Her and Han.

Leo/Karai (TMNT 2012)- Leo and Karai were set up to have, will they won’t they, enemies to lovers, Romeo and Juliet type romance, which was set up in season 1 and carried all the way to the end of the show, one problem, though, splinter finds out she’s his long-lost daughter in the season, one finale and tells the other turtles in the season two premiere, and even after knowing that, Leo still pursues her romantically.

Superman/Supergirl (DC Comics)- originally Supergirl was introduced to add a bit of extra drama to the usual Clark and Lois dynamic with someone who could tempt Clark with the chance to be with someone who he could actually share his superman identity with without fear, naturally this lead to the average plot lines you could expect, jealousy, pining, all the staples, only for the arc to end with the reveal of "Surprise! Turns out I was your cousin the entire time!" Naturally future adaptations just made them cousins from the jump.