Oh wow, that's a search term that sends you down a rabbit hole real fast. Honestly, your results are going to be super dependent on what you mean by 'hot'—like, are we talking high-stakes angst with a side of tension, or more explicit pwp? The main hubs are still AO3 and Fanfiction.net, but the quality and style diverge massively.
AO3 is my absolute fortress of solitude for this ship. The tagging system is a godsend. You can filter for Explicit, sort by kudos or comments, and really drill down into tropes. Some of the most memorable stuff I've found uses the 'established relationship' tag as a jumping-off point for really complex power dynamics, not just smut. There's a writer who does these historical AUs where Katsuki's a samurai and Izuku's a scholar; the heat comes from the cultural and political tension as much as anything else.
Don't sleep on Tumblr or Twitter either, though it's more of a treasure hunt. Writers will sometimes link to their AO3 from a promo post, or host shorter, punchier 'drabble' threads that have a fantastic raw energy. The algorithm is fickle, so following a few big name shippers who reblog recommendations is the way to go. I stumbled upon this one cyberpunk AU thread that was just blisteringly good because someone I followed retweeted a panel from it.
Your mileage might vary a lot based on your tolerance for certain character interpretations. Some writers lean hard into Bakugou's aggression, which can tip from deliciously volatile into OTT if you're not careful. I tend to filter out any 'Dead Dove' tags unless I'm in a very specific mood, but that's just me. The best stories, for my money, are the ones where the physical stuff feels earned by a genuinely rocky, passionate character arc.