Trying to track down stories that merge the world of 'Familiar of Zero' with other universes sends you on a surprisingly specific internet journey. The central premise—Louise's failed summonings leading to Saito being dragged into Halkeginia—lends itself perfectly to crossover scenarios, and the fandom has really run with it. There are entire archives built around the idea of summoning characters from different series, which creates an immediate point of connection for writers and readers who love that initial culture clash and power system collision. You'll find these explorations thrive on platforms that support heavy community tagging and categorization, as that's how readers filter for the specific blend of worlds they're craving.
For large-scale repositories, Archive of Our Own (AO3) is likely your most fruitful starting point. The tagging system there is robust; you can search for 'Zero no Tsukaima' and filter by the 'Crossover' tag, or search directly for pairings like 'Familiar of Zero/My Hero Academia' if you have a particular franchise in mind. FanFiction.net also hosts a considerable number, though its older interface makes browsing crossovers a bit more manual. Don't overlook dedicated forums or spaces within larger anime communities, like SpaceBattles or Sufficient Velocity, where these stories are sometimes presented as creative writing exercises or serialized threads, complete with reader feedback shaping the narrative. These can be goldmines for more experimental or power-fantasy takes.
Beyond pure text, the crossover spirit extends into other media, albeit less formally. You might stumble upon fan-made comics or manga-style doujinshi on artist-focused platforms, reimagining Louise's familiar as, say, a character from 'Fate' or a 'Fire Emblem' hero. Some video game modding communities, especially for strategy or role-playing games, have also created assets or scenarios that nod to this kind of fusion. The search feels less like looking for a single thing and more like tracing a web of 'what-if' scenarios spun from a single magical mishap.