It didn't happen in the show but it SHOULD have happened. It had the ingredients of something good.
FIRST, if we take Suki as eventually a full member of the Gaang, then Toko means that the Gaang all gets paired up with each other. We've got Kataang and we've got Sukka. Toph and Zuko are the outliers, so pair them up. It's what tons of stories do.
SECOND, imagine the political implications. Zuko taking over as Fire Lord means he desperately wants to show the world that the Fire Nation has abandoned its supremacist and imperial attitudes. What better way to do that than by dating somebody from one of the other nations? And Toph comes from the richest family in the Earth Kingdom. It would be a strong sign of Zuko's good faith that he was willing to pair up with one of the Earth Kingdom's most prominent daughters.
THIRD, and most importantly, they'd be good for each other. Toph is the anti-Mai. Mai brings out Zuko's grumpy, self-loathing tendencies. Toph would stomp all over them. Toph would punch Zuko on the arm and tell him to get over himself, which is precisely what Zuko needs to hear, over and over again. Meanwhile, Zuko would be tender and sweet with Toph, which is what Toph needs because of her simmering self-esteem issues. They'd pair up really well.
And now I'm sad all over again that it didn't happen.