The previews I’ve seen make it look like a cross between a black exploitation film and a spaghetti western.
Tarantino used to work at a video store, so I wouldn't be surprised if the saw the 1966 film and was working off that, with an added dose of Skin Game, a 1971 movie with James Garner traveling through the country selling Lou Gossett Jr. as a slave and then helping him to escape, so they can split the proceeds of the sale. You can find a lot of obscure genre movies that probably inspired Tarantino if you google "Django unchained influences."
About Jamie Foxx: I keep hearing how politicians are supposed to be sociopaths. Maybe. But a lot of people in show biz definitely are. Perhaps some day they'll be a biopic about Jamie telling how he messed up his life in one way or another (or overcame his demons, etc.).