UNESCO has an amazing site with a list of some 3000 endangered languages of the world, here:
Not only do I think this is a very interesting possible source for language design ideas, it's also inherently fascinating to me.
What are the cultural conditions under which people stop speaking the native language of their parents? Obviously there are lots of options. Here are two examples:
* There are tons of Japanese-Americans out there who were born during WWII or shortly thereafter, who were never taught Japanese by their parents because their parents didn't want those children to be associated with the enemy.
* There are also children whose parents have been told it would hurt them academically if the parents spoke to them in anything other than English (this is wrong). Consider what that does to a child - it doesn't help them to comprehend English, since usually the English of the parents is rudimentary anyway. In effect, it renders them unable to communicate effectively with their parents, and disrupts all the normal kinds of guidance communication that children need growing up.
Some kids who have a start in one language and then are forced to switch entirely to another never feel like they have "native" proficiency in any language - they're lost in between. The UNESCO database classifies languages based on the way that they are used, whether they're used in the home but not outside, whether they're not used in the home except between older individuals, or whether they're known by an individual but not really used at all.
It's sad for me to think of languages dying - of the richness of cultural heritage and the unique forms of meaning that are no longer expressed when a language disappears. Whether you look at the database out of curiosity, or while looking for language design ideas, it's interesting to contemplate not just how many languages of the Earth are endangered, but how awfully many there are to begin with.