Sokka/Suki: One of Sokka and Suki's children is born blind. At first they don't think it's a big deal -- their two children girls were benders, and Toph is living proof that bending can circumvent a disability -- but as time goes by they realize their third child isn't a bender. Can be set early on or when their kid's older, whatever works best for you.

(Bonus points if you have them living in Republic City.)