Last scene in Django Unchained where Django & Shultz are practicing in the snow?

I didn't catch any reference to future sequels — also because sequels are not decided by open endings or similar stuff, they are decided by the money the film managed to make and the likelihood of a sequel having a similar success (and revenue) —, rather a reference and play on the popular and fixed expression fastest gun in the west.

I can't find an official script to quote from but I guess he does mention the "fastest/quickest" part too.

Now, considering most Western movies are set in... the Western part of the North American continent (with exceptions, some of which quite notable), I think that here what Tarantino was trying to convey was the fact that Django was among the best, if not the best, man with a gun in the South.

This is the only explanation I can think of and I couldn't see anything else on the internet that would suggest otherwise.