I almost put down 'Harry Potter and the Half-Blood Prince' for good after the sixth book came out, because I couldn't stand the way Draco was portrayed. He was a bully and a coward who tried to kill Dumbledore. But then I started reading fanfiction where authors took that same terrified kid and asked, 'What if the pressure cracked him differently?' Instead of becoming a Death Eater, maybe he runs. Or maybe he succeeds, and has to live with it.
The best rivalry-turned-something-else fics I've found don't erase his bigotry or his cruelty from the early years. They use it as the foundation. Growth isn't him suddenly being nice to Muggle-borns because he fell for Hermione. It's him, alone in his room at Malfoy Manor after a raid, finally realizing that the mudbloods he was taught to despise have names, and families, and that their screams sound just like his mother's would. It's a slow, painful unlearning. The rivalry with Harry often becomes a twisted mirror—they're both kids used as pawns in a war, but where Harry had love, Draco had pure, gilded hate. Watching him grapple with that, and seeing Harry's black-and-white view of the world get complicated by an enemy who is also a victim, that's where the magic is for me.
Some writers get stuck on making him instantly redeemable, which feels cheap. I prefer the fics where the growth is messy, where he backslides, and where his rivalry with Harry evolves into a grudging, bitter respect that's far more interesting than friendship.