Kinship terms need work. For one thing I'm thinking "grandma" and "grandpa" should have separate words, and maybe even "aunt" and "uncle" when I think more about it, but the point is: what is the underlying theory here? We do not want Esperanto. There should be some real human logic to the choices.
Beyond the "basic" terms, whatever those are, we need a system for generating the remaining terms for more distant relations. Like, affixes used only in kinship terms. We want to be able to specify...well, actually I'm not even exactly sure what would be relevant because I'm not very knowledgeable about genealogy. But I could certainly find out.