Quentin Tarantino has openly discussed the idea of his characters crossing over: despite the fact that he has two universes in his films (three, actually when you count the Elmore Leonard world of Jackie Brown) there are a couple who transcend even those limits. The Wolf - played by Harvey Keitel - is one (sadly he's also able to advertise car insurance) and Earl McGraw and his son can too.
Don't pick too hard at those rules; Tarantino made them up. But at least it means we might get to see The Wolf again at some point.
So what about the other characters who cross-over or are related by blood across films? Family ties are obviously important for Tarantino (they're a good foundation for vengeance stories after all), so it should come as no surprise that they criss-cross the timelines of his movie universe (and indeed beyond in a couple of notable examples) in a pretty big way...
Relationship: Great Great Great Grandparents John Shaft obviously isn't a Tarantino character, but his relationship to Django and Broomhilda is a notable Easter Egg that Tarantino himself has acknowledged:
"Her and Django will eventually have a baby, that baby will have a baby and one of these days - John Shaft will be born. John Shaft started with this man here and this lady here."
There are also other characters in his universe who are related to other film universe characters: Hattari Hanzo, for instance, is related to the Hanzo lineage from the Shadow Warriors TV series from the '80s and Pai Mei belongs to the Shaw Brothers film series.