Navigating the massive maze of ME crossover fic really depends on what kind of 'multiple fandoms' you're after. I've spent a lot of time on Archive of Our Own; the tagging system is your lifeline. You start with the 'Mass Effect' fandom tag, then filter for crossovers and sort by the number of fandoms listed. It's clunky, but I've found some weirdly specific combos that way, like a 'Mass Effect'/'Dragon Age'/'Star Wars' trilogy that somehow made sense.
There's also a shift in where people post these sprawling things. A dedicated story that tries to weld three or four big universes together often just lives on its own on FanFiction.net, because the author needs more control over the chapters and notes. The comments sections on those old-school sites can be like a mini-community, debating the lore clashes. I had one saved years ago that threw Commander Shepard into the 'Star Trek' universe with a dash of 'Babylon 5', but I think the author abandoned it after twenty chapters.
I've always found that Mass Effect crossovers work best when they treat the other setting as a character development tool, not just a cool backdrop. Take a 'Mass Effect'/'The Expanse' fusion, for instance. Putting someone like Garrus or Liara into the Belter political landscape forces them to confront systems of oppression in a way the Citadel Council's cleaner diplomacy might not. Their established personalities get stress-tested in totally new moral frameworks.
It's less about 'who would win in a fight' and more about 'how does Commander Shepard's black-and-white moral certainty translate to the morally gray, worn-down universe of 'Blade Runner'?' I read one where Shepard was a replicant hunter, and the tension with Tali, who was essentially an AI rights advocate, was phenomenal. The crossover didn't change their core; it just reframed the debate they were already having, making it more visceral.
Mixing starships and spells can go so wrong, but the best ones make it feel inevitable. The 'Dragon Age: Origins' crossover where Shepard is a Grey Warden recruit from a crashed shuttle is probably the most polished I've read—it treats the tech like strange magic and the darkspawn like a new kind of Reaper threat. That author really nailed the party banter vibe from the games.
There's another, less famous one crossing with 'The Witcher' that just sticks in my head. Shepard as a witcher-school graduate, biotics explained as a chaotic mutation. It's grimy and philosophical in a way the source material isn't, but it works because both series are about monstrous things and the people who hunt them. The prose gets a bit clunky in fight scenes, though.
I tend to avoid the high fantasy like 'Lord of the Rings' blends; the tone clash is usually too severe. But give me a setup where the Normandy crew lands in Skyrim and has to deal with the Thalmor? I'd read that mess in a heartbeat.