Since R3, when playing as the Nabataeans, if you control Dedan, Rekem, Bostra, and Tadmur, and Mauryab is not owned by you, the campaign script awards you 1000 mnai every 4 turns. This simulates the Nabataeans gaining and reaping the rewards of the trade monopoly from southern Arabia to Hellenistic markets. Historically, middleman nations in the trade scene gained obscene wealth, and would do anything and everything to protect their strangleholds. You've seen it with the alt-history Nabataeans in EBII, but you can also read about how the Parthians did all they could to keep the Roman Empire and the Han Dynasty from establishing direct trade relations in order to protect their position as the intermediaries of the Silk Route.
Now, onto the gameplay. The 1000 mnai every 4 turns is much appreciated for a burgeoning Nabataean kingdom surrounded by superpowers, but still a little on the low side. I would not only increase the monopoly taxes from 1000 to 3000, but also include a small boost to faction relations to the Ptolemies and Seleucids every turn that the monopoly taxes are rewarded. This is assuming that you are at least neutral with either of the two nearby superpowers. It's not enough to make them tolerate you pulling off bribery and other underhanded diplomatic maneuvers, but it should at least make them respect you enough to not regularly threaten you and by extension the Red Sea trade.
Furthermore I would love to see this feature expanded to showcase other historically important trade routes/monopolies. The Amber route from the Baltic sea to the Adriatic and Carpathian Mountains is one such candidate. The Persian Gulf trade between the Hellenistic destination of Antiochea Sousianes and the rest of India, tightly controlled by the narrow entrance to the gulf between Alexandreia Karmania and the settlement right across from it in Arabia, could be another one. The last historical example are the Straits of Gibiltrar. Exotic goods from as far away as the British Isles would end up in the Mediterranean, courtesy of the predominant Carthaginians and brave Celtic merchants.
To go even further could the campaign script also include hypothetical monopolies? For example, controlling both sides of the Hellespont would award you 3000 mnai every 4 turns, though these taxes stem purely from bullying merchant ships trying to pass through the easily-controlled Hellespont. This would also decrease your factional relations with KB every 4 turns... an acceptable political loss for economic domination of the Aegean and Pontic Sea.
To wrap things up, popup messages explaining the historical background of each monopoly could pop up for the first 4 activations of the mnai bonus, just enough times to drill the history lesson into the player's head
Anyways, kudos to the EBII team for adding this feature. It's a great example of teaching you history through gameplay.