Well, if you're poking around the 'Mother 3' fandom for Lucas and Kumatora stuff, you've probably noticed it's not the biggest ship out there, but that kinda makes the themes people do explore more interesting. A lot of stories lean into the 'found family' angle, for obvious reasons. They're both kids who've lost their original families in pretty brutal ways, so writers love putting them together in scenarios where they build something new—running away together, creating a home in the Nowhere Islands, that sort of thing.
Another huge one is the 'post-canon fix-it,' because let's be honest, that ending wrecks everyone. Authors will have them surviving the aftermath, dealing with the collective trauma, with Kumatora's tough exterior slowly crumbling as Lucas tries to help her process things. It's all about vulnerability hidden under bravado.
You also get a fair bit of 'canon divergence' where maybe Kumatora joins the party earlier, or they have more one-on-one moments during the journey. The dynamic is usually her teaching him to be stronger, both physically and emotionally, while he teaches her that it's okay to not be tough all the time. Less about romance, more about mutual healing and growth, which feels very true to the game's spirit. Honestly, the best fics I've found aren't even tagged as romance half the time.