Let's be real, the most common trope for Changbin and Felix from Stray Kids has got to be 'Changbin is obsessed with Felix's freckles.' I've lost count of how many fics start with Chan noticing them under the practice room lights and just... short-circuiting. It's such a specific, visual detail that really roots the story in their real-life dynamic.
A close second is the classic 'Chan is the overworked leader, Felix is the sunshine that forces him to rest.' It's a comforting, foundational dynamic for the fandom. You get Chan buried in his laptop, skipping meals, and Felix just appearing with a plate of fruit or dragging him to the couch for a cuddle. It plays into Felix's caring reputation and Chan's known dedication, amplifying those traits into a narrative. Sometimes it veers into hurt/comfort if Chan actually gets sick, which Felix then nurses him through.
Then there's the whole 'Hyung line is protective of Felix, especially Chan' trope, which often overlaps with the others. It's not unusual to see fics where Chan is subtly but fiercely guarding Felix from outside negativity or even from the other teasing members, framing that protectiveness as something deeper. The tropes work because they feel like extensions of what we see on screen—amplified, sure, but built on a recognizable core of Chan's leadership and Felix's bright, affectionate nature.