As far as the re-institution of legal slavery, I think it could plausibly develop in most any country. Let's take the U.S. Let's say a racist, nativist xenophobe is elected President. Obviously that won't happen any time soon, so this is clearly set in the far future. Let's call this President "Ronald Crump." Late in Crump's second term there is a national emergency--a significant terrorist attack on American soil or perhaps a more or less conventional war is begun with a foreign nation. Crump essentially declares martial law for the duration. The Constitution is amended so Crump can have a third term in office, and at the end of that, is unanimously proclaimed President-For-Life. As part of his many sweeping revisions and reforms, the justice system is overhauled and streamlined. Mandatory Minimum sentences are extended and broadened to encompass many more kinds of crime. For instance, Dissemination of Disinformation, the "Fake News" statute, is a federal offense, with a mandatory minimum sentence of five years. Journalists or reporters of any kind who file stories seen as hostile or unflattering to the Crump administration are convicted under the statute, and given long prison terms. Soon, anyone seen as opposing the administration is jailed.
Jails and prisons are privatized. As profit oriented institutions, prison administrators no longer permit inmates to simply serve out their time in cells or in a prison yard. Rather, their labor is sold to the highest bidder, to replace the immigrant labor that had been doing the difficult, monotonous, dirty, even dangerous work that American citizens had been unwilling to do. Prisons become more profitable by reducing their overhead and expanding their productive capacity. The work force is expanded by more laws and longer sentences. Once again it is common to see chain gangs along the roads and working in the fields.
The Supreme Court, now packed with justices who are as reactionary as the man to whom they owe their fealty, rule in Barbaria v United States that corporal punishment does not necessarily fall under the "Cruel and Unusual" prohibition of the eighth amendment. "The whipping post, the stocks, the pillory, were in common use during the time of the Founding Fathers," writes the Chief Justice in a unanimous opinion. "It is absurd to suggest that the original intent of these men was to prohibit the use of such devices to further the cause of justice in a well ordered and disciplined society. The application to this court by petitioner for stay is denied, petitioner may be whipped."
Perhaps this is not strictly slavery. Let's call it "Disciplinary Institutionalization and Rehabilitation."