Defining 'fixi' as that niche where gritty, systemic fantasy meets meticulous worldbuilding, I'd point straight to Brandon Sanderson. His whole Cosmere project is basically a masterclass in fixing a broken world—you've got magic with rigid rules, societies rebuilding from cataclysms, and characters who are often engineers or scholars at heart. 'The Stormlight Archive' is the obvious entry, with its focus on healing a shattered land and broken people.
Joe Abercrombie deserves a nod too, though his approach is less about fixing systems and more about fixing—or more often, breaking—people. His characters are constantly trying to mend their own flaws or the messes they create, usually with bloody, tragic results. It's fantasy where the repair work is deeply personal and morally grey.
Then there's N.K. Jemisin, whose 'The Broken Earth' trilogy is the ultimate fixi premise: the world is literally ending on a cycle, and the story follows those trying to understand and maybe stop it. It's less about crafting new magic gadgets and more about sociological and geological repair, which feels incredibly fresh.