I'm honestly surprised the 'quirkless Izuku trains under Stain' idea hasn't blown up more than it has. You get this intense, morally grey mentor-student dynamic where Inko is initially horrified but then has to grapple with the fact her son is learning brutal but effective skills from a vigilante murderer to achieve his dream. The tension between her protective instincts and seeing Izuku become more capable and confident is a goldmine for angst and slow-burn family drama. It often leads to Izuku developing a more pragmatic, analytical fighting style rather than just overpowering opponents, which feels truer to his character.
I've seen a few where he uses blood-based 'paints' to mimic artistic styles for combat, which is a fun, specific twist. The best ones don't just make him a mini-Stain; they explore how Izuku's innate compassion clashes with Stain's extreme ideology, with Inko caught in the middle trying to understand this terrifying path her son is on. It's less about the power fantasy and more about the psychological cost.
That said, a lot of these fics get abandoned after the training arc, which is a shame.
I read one recently where he was a chronic pain patient and Inko had to navigate that daily reality while he still tried to be a hero, which felt incredibly grounded.