"Innovation" is the wrong word for a practice that was already thousands of years old on the European continent and had huge bodies of jurisprudence and precedent attached to it, in Latin, Greek, Persian, etc. law, as well as in oral traditions elsewhere. But it's true that it's important not to look at slavery anachronistically, as if the alternative to slavery was the egalitarian status of a modern citizen of a state declaring the value of universal human rights (that very contrast was what gave U.S. slavery its distinctively horrible brutality, in historical terms). In Roman law the most sanctioned and canonical reason for someone to be a slave was that they were a captive in war, and thus that they were a slave instead of being dead, not instead of being free. In a sense they were "dead", having been reduced to a thing, a piece of property, and only emancipation would restore them to life. (In some ways this was an aspirational fiction, since most slaves during Roman times were actually defaulted debtors, not captives in war).
Roman slavery was (like US slavery) distinctively extreme in its reduction of persons to objects; in many societies (Vikings included) slaves did have some rights. In many implementations of slavery, slaves could marry, own property (even own their own slaves), and protest cruel treatment; the Roman (and US) hell in which a master could separate families, sell children, or even put slaves to death essentially on a whim, was unusual (though Romans, whose slavery was purely economic and unrelated to racism, did a good deal less of selling their own children). In Dark Ages societies not under the remains of Roman law, slaves were often much closer to vassals, bound to serve, but seen as persons, not things.
All this brings me to the question of what a playbook for The Slave would look like, particularly if the slavery was closer to Roman-style slavery. Perhaps all the initial options could begin "You have no rights, and yet you..."
(David Graeber's Debt: The First 5000 Years has a bunch of interesting things to say on this topic)