I've spent way too many hours scrolling through tags on Archive of Our Own for this exact premise. The best ones, in my opinion, are the ones that lean into their shared understanding of being prodigies who had to grow up too fast, but in opposite directions. Zuko's honor-obsessed discipline versus Toph's rebellious, earthbending freedom creates a fascinating friction. A lot of fics just put them together as a quirky odd couple, but the hidden bond is deeper—it's about rebuilding a sense of home and family after losing theirs.
One standout for me is 'The Earth King's Favorite Tea Server' by bananabreadboy. It's post-war, with Zuko working incognito in a Ba Sing Se tea shop and Toph, as the head of the metalbending police, stumbling upon him. The bond isn't romantic at first; it's built on shared silences and Toph's unique perception of his firebending through the earth. She can feel the controlled heat of his flames in the floorboards, a detail most authors miss.
Another angle is through the 'Gaang as family' trope. 'Foundations' by melonlordapologist explores Zuko teaching Toph about Fire Nation architecture's stability, and Toph teaching him to 'listen' with his feet, a metaphor for grounding himself. Their hidden bond is less about whispered secrets and more about practical, wordless support. It feels earned, not forced.