The dynamic between the Powerpuff Girls and the Rowdyruff Boys seems to endlessly fascinate writers, and a handful of patterns definitely keep popping up. There's the whole 'sworn enemies to reluctant allies to lovers' arc, which practically writes itself given their origins. I've seen a ton of fics exploring the mirrored creation myth—Blossom and Brick grappling with the burden of leadership and fire-based powers, Buttercup and Butch bonding over their aggressive, competitive streaks, and Bubbles and Boomer connecting through a gentler, more artistic side hidden beneath the surface.
Another massive one is the post-canon 'what now?' scenario, especially after that 'The Powerpuff Girls Rule!' special. They're all grown up, the world is saved, and the old rules don't apply anymore. That's when you get the coffee shop AUs or the 'forced to work together on a joint Townsville task force' premises. The 'villain redemption for love' trope is huge for the Boys, with Buttercup often as the skeptical hardliner who takes the longest to believe Butch has changed, while Bubbles is the first to offer Boomer a chance.
A more niche but fascinating angle I've stumbled upon is the 'body swap' or 'power swap' story, where they're forced to literally walk a mile in each other's shoes. It's a clever way to force empathy and deconstruct their prejudices. You'll also find a surprising amount of 'amnesia' fics, where one side loses their memory and the other has to navigate this weird, tense relationship with a blank-slate version of their rival. The tropes are familiar, but the specific flavor of superheroics and 90s cartoon logic gives them a unique spin.