A fic that truly got me was one where Sirius was wrongly imprisoned, but instead of Azkaban, he's secretly detained by the Ministry in a hidden lab where Snape is a potions consultant. Their dynamic isn't romance at first—it's pure, simmering hatred undercut by Snape's reluctant recognition that Black isn't the monster he thought. The slow realization that they've both been Dumbledore's pawns, just in different ways, hits hard.
I'm always drawn to 'what if' scenarios that peel back their schoolboy rivalry. One theme I love is forced proximity during the First Wizarding War, maybe on a joint Order mission gone wrong. The tension between Snape's Occlumency walls and Sirius's raw, impulsive magic creates such a volatile atmosphere. You can practically smell the damp leather and old potions ingredients.
What makes these themes work is how they invert expectations. Snape's meticulous nature clashes with Sirius's chaos, but under extreme stress, those differences become complementary. A great story uses the magical world—legilimency, unbreakable vows, enchanted objects—as a pressure cooker for their relationship, whether it turns antagonistic, respectful, or something more.
Endings where they don't magically become friends feel more authentic to me. A grudging, silent understanding as they watch Harry from afar sometimes carries more weight than any dramatic reconciliation.