Archive of Our Own is my primary hunting ground, without question. The tagging system is what seals it – you can filter by ‘Vasto Lorde Ichigo’ as a character tag, ‘Hollowfication’ as a trope, and even pairings like Ichigo/Uryū or Ichigo/Grimmjow if that’s your jam. It cuts through the noise. You’ll find the ambitious, novel-length AUs where Ichigo’s transformation is the inciting incident for a whole new power balance in Hueco Mundo, not just a one-chapter power boost.
Some of the most memorable takes live there, like a story where his Vasto Lorde state is semi-permanent and he has to navigate a wary Soul Society, which is a concept you rarely see executed well elsewhere. The quality control feels higher; writers there often treat it as a serious character study rather than pure power fantasy. I still check FanFiction.net out of habit, but sorting through the untagged, inconsistently summarized fics for a specific concept like this feels like digging in a landfill compared to AO3’s curated library.
My backup is surprisingly specific: Japanese fanfiction sites like Pixiv or Syosetu, using machine translation. The cultural interpretation of ‘Vasto Lorde’ and Hollow nature can be radically different, more psychological and body-horror adjacent. The prose often suffers in translation, obviously, but the core ideas are sometimes fresher than the English fandom’s usual tropes.