The inquiry is what are the privileges and immunities of citizens of the several States? We [royal "we"] feel no hesitation in confining these expressions to those privileges and immunities which are fundamental; which belong of right to the citizens of all free governments, and which have at all times been enjoyed by citizens of the several States which compose this Union, from the time of their becoming free, independent, and sovereign. What these fundamental principles are it would be more tedious than difficult to enumerate. They may all, however, be comprehended under the following general heads: protection by the government, with the right to acquire and possess property of every kind and to pursue and obtain happiness and safety, subject, nevertheless, to such restraints as the government may prescribe for the general good of the whole.Justice Washington provided this influential definition of the "privileges and immunities" of citizenship as applied to the provision in Art. IV. Madison in Fed. 54 noted that society in effect artificially limits the natural rights of slaves and once slavery was removed, they would be in equal standing (at least for purposes of representatives). This suggests the possible reach of the Thirteenth Amendment: once slavery was removed, everyone would have the basic rights of citizenship or at least freedom (the former perhaps raising issues of voting and such).
And, the revolutionary federal enforcement article would provide the central government the power to do what was primarily left to the states: enforcement, even against private action, of violations. The breadth of this principle is seen by those who connected the 13A with the Declaration of Independence with its broad protection of natural rights and reference to government "securing" such rights. The collection of essays touches upon this though it for some reason barely references the Northwest Ordinance, which has an early form of the text and the editor of the work has written another book and other writings supporting the open-ended approach, also writing a book on the DOI itself.
The potential of constitutional provisions is seen in various areas from federal powers to rights found in the Bill of Rights, text provided without some express limit of how far to take it. There were cases, however, where further action was taken to limit certain potential paths. For instance, using the Bill of Rights to help ensure potentially open-ended federal powers were not taken too far. The Fourteenth Amendment also has this function. Some thought the 13A gave the federal government to broadly define and protect (even against private action -- it is a rare case where a federal constitutional right is so protected; another would be travel) rights, including the nature of federal citizenship. Controversy led to the ratification of the Fourteenth Amendment, which covers such ground.
The question then remains what the 13A itself covers. The basic idea is to determine the reach of the "badges and incidents of slavery,"basically the "penumbra" (again showing the Griswold concept is fine, even if you find the word funny) of the bare prohibition of slavery and involuntary servitude. Current precedent gives Congress some degree of discretion here [thus, one might have certain rights arising from this amendment only as a result of legislation, while something like freedom from bondage is so clear that the 13A is "self-executing" on the matter], including banning private racial discrimination in housing, a concurring opinion suggesting the possible further reach here. Lower court opinions have more recently upheld federal hate crime legislation and the Supreme Court itself noted "race" at the time included Jews and Arabs, thus some degree of religious discrimination applies too.
We are told that a turn of the 20th Century opinion (with two justices dissenting, Justice Harlan finding a friend) that held that interfering on account of race to enjoy employment was not covered was wrongly decided. Harlan dissented in The Civil Rights Cases and Plessy, in part holding state supported racial discrimination, including regarding "private" businesses (if involving "public" accommodations) were a type of "badge" of slavery. Yes, it was present even in free states, but the overall idea of treating blacks differently arose from the same basic mind-set that allowed slavery. Ditto things like miscegenation laws though even Harlan accepted that sort of thing, though some did not.
Congress (states always having the power to individually address it) eventually used the Commerce Clause to handle this sort of thing, but a few justices at the time this came in front of the USSC were a bit wary of merely relying on that sort of thing. It was a bit too materialistic in a fashion to treat people as merely aspects of commerce (a bit ironic too) when the basic principle of racial discrimination provided a higher value. Public accommodations had enough state action to meet 14A requirements. The book also noted that unions in the early 20th Century also fought with some allies who did not want to rely on the 13A (true contractual freedom, given the power of big business, required union freedom) over the Commerce Clause. The more pragmatic approach had immediate value, but long term, did not adequately do the job.
The last point again opens up the question of scope. The basic core of the amendment is freedom of contract, thus it was one of the few areas that even the "Lochner Court" honored respecting civil liberties writ large. The book, however, noted that in time such a let's say atomistic view of things was seen as too limited. During the New Deal, there the federal government recognized that the 13A included some degree of protection of workers beyond the mere ability to leave employment without being arrested (peonage laws guarding against this even if you are in debt, which was a standard thing for the masses under let's say Southern shareholder systems). Some minimum degree of protection of workers were necessary too. A later essay notes how this also applies to immigrant workers, even those undocumented. The 13A applies to all. A situation where the government, and even private parties under its jurisdiction, is limited even when citizens are not involved or even home soil (e.g., no slavery by the U.S. government on the high seas).
This suggests the potential broad reach even if we stick to the core concerns of the amendment: free labor, which is not merely a matter of race discrimination. Race clearly is a core issue here, particularly the social definition of "black," though even in the 1860s there were matters involving Asians, Latinos and Native Americans. So, things like racial profiling (e.g., the use of the Exclusionary Rule here too) and hate crimes also raise 13A concerns, again the potential reach to private parties showing why the 14A is not enough here. Use of Confederate flags on state property is also argued to be a "badge" of slavery by some. And, since later amendments add gloss to earlier ones, this might even possibly help deal with concerns that hate crime laws interfere with the 1A.
The most expansive reach of the amendment would go beyond labor and race relations to deal with open-ended protections of freedom as a whole. There are various levels here too though. I linked to a form of the chapter in the book arguing (though Dawn Johnsen even citing the argument in a footnote helped block her nomination to the OLC -- still rankles) that abortion freedom is a 13A matter. I think it is a reasonable argument: you are being required to labor (quite literally) for another, opting out is allowed even when you consent (if that word even works in many cases here) and lack of freedom over reproduction was an evil of slavery. Another area would be spousal or child abuse. Proper legislative responses aside, this reasonably can be seen as a sort of "slavery."
Finally, there is an open-ended idea that various basic aspects of freedom, beyond direct or indirect coercion of the sort cited or "badges" of slavery like hate crimes or flags, such as freedom over your personal life. Thus, just as some in the 1860s noted slaves could not make basic family decisions, who you marry (including same sex partners) would have 13A implications as would IVF choices or making basic health decisions. Personally, though I see the logic there, I think that is better seen as a 14A matter, which addresses open-ended rights citizens and in many cases all people enjoy, while the 13A has a certain more restrictive reach.
Slavery would help define some of these rights as a sort of anti-canon to determine natural rights by their absence. A closer call would be the reach of federal hate crimes as applied to non-racial categories, such as sexual orientation or sex/gender. There, a healthy application of the 14A, including accepting states not properly protecting rights equitably (see, e.g., Breyer's dissent in U.S. v. Morrison) would also help. The matter of truly free labor would be to me the most fruitful area, especially in this time of anti-union efforts, including proper treatment of immigrants.
As noted, the collection covers much of this ground, though I found a few of the essays a trudge and not quite as comprehensive view of the history of the amendment (though a few essays cover the ground) as I would have liked. Also, the editor does have a book he himself wrote that covers the amendment is regular book form for those interested. Looks fairly good though I didn't have an interest as such to read it as well.