One of the fun things about this project is that I am learning lots of random but useful bits about the ancient Roman world. I've been spending some time looking closely at Trajan's coinage and wondered about the significance of the caduceus (a symbol that I--wrongly--had always associated with medicine and healing). It turns out that, in fact, it is the staff carried by Hermes/Mercury, the messenger of the gods. In the context of the iconography of Roman coins, it signifies commerce and trade. Thus, it appears in association the image of Mercury, Felicitas, cornucopiae, and other symbols of trade and abundance. In the case of Trajan, it seems to have multiple valences: most obviously, it celebrates the emperor's role in ensuring a steady supply of corn for Roman citizens. More broadly, it acknowledges Trajan's activities as builder of the roads and bridges that support trade (by facilitating the transportation of both people and goods across space). Trajan understood very well the close connection between military conquest and the Roman economy; and one of the things he famously did was a tremendous amount of road construction in Italy but also in provinces that he annexed. One feature of Romanization under Trajan was provincial participation in the larger Roman economy.
Something I am thinking more about: to what extend might Trajan have been imagining the Eastern Mediterranean provinces as something other than a tax base? To what extent were his last campaigns about setting up a system of management in which the much more wealthy eastern provinces were more fully integrated into the imperial Roman economy? In fact, this never happened and the more prosperous East always remained separate from the economically fragile West. The West had been in trouble for a long time, since the last century of the republic. The wars of Caesar and Pompey were a temporary solution and provided significant influxes of wealth; likewise, Trajan's conquest of Dacia. But, really, there was nowhere else in the west to subject to the traditional "conquer and plunder" model of economic enrichment. Really, for the Empire to be sustainable, it needed to develop a new economic model, one that treated the eastern provinces as something more than just a source of tax revenue. It's very possible that Trajan recognized this.