In this world, "people" come in all shapes and sizes. Every race is originally of human stock and is the result of intentional experiments by wizards over the millennia...and favorable (or unfavorable) mutations.
With that in mind, feel free to choose any ancestry (except Half-Giant and Ooze/Construct) and any background (except "Made a BAD Choice").
You can take the mechanical benefit from any ancestry or background and create your own flavor/explanation for it.
I've extended the selection available from the Core PDFs to contain additional non-Core options.