The Adjudicator in The Continental states that it PREDATES (holy shit) the roman empire which is just downright diabolical and I think it started off more as like this group of crime, doing petty things before it eventually started evolving with the modern times. I personally headcanon the High Table’s roots being formed during the time of the Roman Republic which is one of the earliest examples of democracy in history(?) and because of politics and the vast amount of things that happened during that time, I definitely head canon the High Table being formed at that time as nothing more than just maybe a simple rebellion group that does crime but with not much power, but quickly gained that control.
I think as crime came to be, around the 19th century, mafias started forming, primarily the Sicilians (aka the Cosa Nostra) I think it was mostly in Europe, or maybe just in Italy, placing us at three seats with the Cosa Nostra being the first, and the the ‘Ndrangheta(Calabria) and the Camorra(Naples) following after. While the Yakuza and the Triads did exist during those times, I don’t think there was international communication that would be available at that time, but I think the Triads would be added shortly after and then the Yakuza, then placing us at five. It’s only then when things started modernizing that I think the other seats would be formed, such as the Cartels, and then the Bratva which was surprisingly started around the 50s lmao. I thought it was started in the 1800’s 😭
To make an entirely new seat would depend on if a mafia is big enough. The cartels for example, are big as hell both in members and in influence. Which goes the same for the Ndrangheta, the Camorra, Cosa Nostra, the Triads, the Bratva, and the Yakuza. BUT if there were a new seat, I would theorize it would be the Italian-American mafia in New York as they already have a vast amount of allies overseas in Italy as well as enough families under them