Notice in the series ender, The Duke hurries to hug Toph (hinting affection), despite only meeting her during The Invasion. They were not shown interacting much during the episodes following the failed invasion, but that's likely due to the story focussing on everyone's "Field Trips" with Zuko.
And Toph hugged back. If I'm not mistaken, that's one of the very few times Toph hugged a person, as her only hugs were group hugs with Team Avatar.
And has anyone else noticed the similarities between The Duke and Lin?
People can't hug each other without setting up the possibility of an intimate relationship?
I would prefer to think Toph found a hard working, charming, somewhat goofy man for her own later in life. Don't like hte idea that everyone we saw paired together in the original series has to stick together. Yeah, some of them should, like Aang and Katara and Sokka and Suki, but it's not realistic for this to stretch to include everyone.
lin's father and toph's husband may not be the same man. also, maybe toph never got married. just sayin' it's a possibility.
personally, i think she married a character we've never seen before, and that her husband is her daughter's dad. look at amon - everyone was guessing who he was, but bryke made up a whole new character for the man behind the mask.