Katsudeku's always had this really interesting push-pull thing in fanfic, and you can see it in the themes that keep popping up. A lot of writers are drawn to the 'rivals to lovers' arc, but specifically the ones where Bakugo's anger gets reframed as this deep-seated, confusing affection he doesn't know how to express. You'll find a ton of stories exploring his guilt over the bullying, with Izuku either fiercely forgiving him or needing a long, angsty journey to get there. The whole 'hurt/comfort' tag is massive for them, often with Izuku getting injured on patrol and Bakugo being the one to panic and realize his feelings.
Another huge one is the 'domestic AU', but it's rarely just fluffy pancakes. It's usually paired with something like 'Pro Hero Domesticity', where they're forced to live together for security reasons or share an apartment after graduation. The tension comes from navigating their competitive dynamic in a shared space—who does the dishes, who wins at video games, that sort of thing. It's a framework to soften their edges without losing their core personalities. Soulmate AUs are popular too, but the marks are often aggressive or tied to their quirks, like Izuku getting Bakugo's explosion scars on his skin.
Lately I've noticed more fics playing with power dynamics in a literal sense, like 'Quirk Accident' or 'Forced Bonding' scenarios where their quirks get entangled. It's a way to force physical proximity and emotional vulnerability. The themes aren't really that different from other popular M/M ships, but the execution feels distinct because their history is so canonically visceral. You can't write them without addressing that past, so even the fluffiest coffee shop AU has this underlying layer of reconciled conflict. It's what makes reading them so satisfying, even when the plots get repetitive.