I also recommend taking some inspiration from the Roman Republic/Empire. They declared aggressive expansionist wars in the name of self defense, the highest ranks of political office (Consul) were only achievable through military service, successful military campaigns were rewarded with personal wealth and political power, and new Consuls were elected every year so the system incentivized aggressive military campaigns. Also the Roman religion was highly syncretic, often adopting foreign practices into the worship of the Roman pantheon as a means of assimilating conquered peoples into Roman culture. Not to mention the old Roman tradition of unending civil wars.
Something I don’t really like in a lot of D&D settings is that there are a ton of mono-racial empires. In the Forgotten Realms the Dwarves had an empire, as did the Elves, as did the Dragons, as did the Humans, as did the Giants, and so on. The Roman Empire was started by a single city, but grew into one of the largest and longest-lasting empires in history. It became multicultural and multi-ethnic over the centuries. There were Emperors from all over the empire (Italy, Hispania, Africa, Syria, Greece, etc). A D&D equivalent could have started as a single city mostly of one species (Human, Elven, Orcish, Dragon, or whatever) but eventually have leaders of any kind race. Maybe Humans started the empire and it’s government was filled mostly with humans in the early days, but after conquering new land and assimilating the locals they had Elven, Dwarven, Halfling, and Orcish members of the government (Senators, Generals, Consuls, Emperors, etc).
Or maybe the Empire is a bit more xenophobic and discriminatory, but there are ways for people of other species to gain power. For example maybe there’s an Empire that was started by Dragons and they do assimilate those they conquer into their culture/religion, but have laws preventing those without Draconic heritage from becoming members of the government. So Kobolds, Dragonborn, Half-Dragons, and Draconic Sorcerers have social mobility, but other races don’t. This could incentivize people to try and become more Draconic so they or their descendants can gain more political power. So maybe a rich human will pay a dragon for their blood and then feed that to their children in hope that they’ll become a Draconic Sorcerer, or they’ll pray to Bahamut asking to be transformed into a Dragonborn, or will even pay a dragon to have a child with them to allow for their descendants to have better lives in the empire. This could create an interesting story for Draconic player characters in this setting, as they would have ties to this empire and thus a complicated relationship with their heritage. They are simultaneously the heirs of an aggressive expansionist empire and its victims, affecting how others treat/perceive them.