I'm surprised no one (AFAICT) has attempted a family tree of all humans. There is an obvious demand for this information because many commercial services exist. Users are paying to upload their personal genealogical data to proprietary for-profit silos. Yet this data would be much more productive in an open system with user data from all services.

You could seed the database with famous people's family trees from Wikidata. The Mormon church also has lots of genealogy data that (perhaps :) they might share for not-for-profit use.

The biggest challenge would be preventing trolls and spammers from uploading false data. I've sketched out some rough ideas where family links can be "thumbs up'd" bidirectionally by people on both sides of the connection, but not necessarily the immediate people.