Fanfiction.net's a total mess for navigation, but I swear it's got this weird archival density you don't find elsewhere. For Bleach crossovers with real worldbuilding teeth, I'll filter by fandom crossovers and sort by favorites, then immediately jump to like page 5 or 6. The front page is all 'Ichigo in Westeros' power fantasies, which, fine, but the good stuff is buried. There was this one, 'As N Approaches Infinity,' that crossed with 'The Melancholy of Haruhi Suzumiya' of all things, treating the SOS Brigade's reality-warping as a form of spiritual particle manifestation that was breaking down barriers between worlds. The writer actually used Quincy lore to explain it. You gotta dig past the obvious Naruto or One Piece crosses, though sometimes you find a gem like a Bleach/'Fullmetal Alchemist' fic that treats the Homunculus as souls bound by alchemical equivalent exchange, forcing the Shinigami to confront a completely different system of soul law.
Honestly, my most consistent hits have come from SpaceBattles or Sufficient Velocity forums. The culture there leans toward technical discussions, so even in the creative writing sections, authors feel pressured to explain their magic system integrations. You'll get threads where people are debating whether a Zanpakuto could conceptually cut through the ontological boundaries of the Warp from 'Warhammer 40k' or how Hollows would interact with Cursed Energy from 'Jujutsu Kaisen.' The brainstorming in the comment sections sometimes spawns whole new fic ideas. It's less about shipping and more about the mechanics of the clash, which is exactly where creative worldbuilding thrives. I found a stellar Bleach/'Lord of the Rings' story there that treated the Rings of Power as artifacts created by a precursor race of soul-weavers, making the entire conflict a secret spiritual war. You just need a tolerance for forum formatting and people arguing in the middle of chapters.
Archive of Our Own is trickier. The tagging system is superior, but you have to be clever. Searching just 'Bleach Crossover' and 'Worldbuilding' isn't enough. I look for tags like 'Alternate Universe - Fusion,' 'Philosophical,' or even 'Science Fantasy.' The real creative ones often get tagged with very specific lore terms, like 'Original Kido' or 'Hollow Biology.' Sometimes the best worldbuilding isn't in a huge fandom cross, but in a tiny one where the author has to build bridges from scratch. I read a Bleach/'Dresden Files' fusion once that was practically a thesis on comparative spirit hierarchies. It's out there, but it's rarely on the first page of results.