The Hann Empire isn't really an empire in the traditional sense, as it doesn't have a ruling emperor. Politically, it is more of a federation, with its member states bound together by treaties that are mediated by a Grand Council which is overseen by the Raven King of Armes.
Chaired by the Raven King of Armes, the council consists of representatives of all the states. There is a formula for deciding how many representatives each state can send , and that is applied rigorously by the Raven King of Armes, a decision that can only be overturned ob appeal to the full council. Members of the Council are often junior members of ruling houses, associated nobles or priests of a deity interested in the trade portfolio.
However, it is also inclusive. Representatives of non-human races and leaders of non-Hann states are welcomed into the Empire, so long as they can agree sensible trading terms and contracts. While inland states can be quite insular, coastal towns and cities are culturally vibrant, and resonate with the sounds of many languages and cultures.
However, there are a number of other factors that help keep everything running smoothly - or at least relatively smoothly.
The Green Faith holds sway across the whole of the Empire. While different areas have slightly different versions of the Faith, they all agree that they follow the same philosophy. The world and nature is important, so is a balance between the different aspects of a societies culture. On top of that, each main religion group has a deity that shares many of the same philosophies. Some are even seen as powerful spirit representatives of The Green Faith. The Green faith is, perhaps, the only true universal faith - certainly it is the only universal faith among the states of the Empire. The Raven King of Armes, who chairs the Grand Council is a high level Druid of the Green Faith.
The Old Gods have some influence all across mainland Hann. They were once the patron pantheon of the Hann people, and they still have something of a following. Just as importantly, they are the Gods behind many of the traditions and holidays that are practised and celebrated across the mainland empire - and this reminds all of the main parties that they have common origins and interests.
Principle deity of The Tannery States, Abadar is a god of civilisation and trade, who believes that wars are bad for both. His High Priest is very influential, and pushes the Tannery States to fully embrace the concept of the Empire, and the trade it can bring in. Therein lies growth and prosperity for all.
The neutral good leader of the largest pantheon, Pelor wants the best for everyone, and his clerics all support this framework that lets everyone live in peace and prosper.