We run a sports history site and occasionally we run into issues where a person was born in Prague, but in 1887, and we wish to display the correct country at the time of their birth rather than at the current day.
We have used geonames extensively, but we need a historical geonames, where we could enter a current territory and a date and find the correct historical representation of that location. I realize that things like city annexation etc might complicate this somewhat, but for now I'd be very happy with source that lists a city's country over time.