Games need conflict. And conflict usually involves enemies.
Since pulp is an inspiration for Age of Air, I’ve been looking at Pulp HERO. What’s the classic pulp villain? The Nazi.
So, we’ve already got a setting with zepplins. How do we inject 500 cc of National Socialism?
Ok, we need a society, bent on world-domination. It values martial traditions, loyalty, discipline, and strength. It’s a conquering society. Since I picture the campaign era being an age of peace, perhaps they’ve just gotten done conquering some neighboring territory- there was a short war, with the ‘good guys’ supporting the defenders, but we lost. The bad guys say that’s all they wanted, but true heroes (read: the PCs) know better, and work against their schemes. (This also clenches them as a threat- if ten years ago, the bad guys won the Great Air Battle of Such-and-Such, people will sit up and take note when evil zepplins fill the skies.)
Their leader is personally charismatic and has the complete loyalty of his people; he likely attained his position through less-than-legal means. His inner circle is a rogues gallery of unusual types, ripe for the picking for ‘boss enemies.’ They wear awesome looking black leather uniforms.
When you have pulp nazis (or hell, even real-world nazis), you have gonzo mad scientists. This somewhat clashes with the fantastic idea of the setting, but perhaps this society instead has lots and lots of artificers. They’re mechanized; really really mechanized. They’ve got the best zepplins, the best crossbows, and the best retractable butterflywing harnesses. (Actually, their equivalent should probably be ravenwings. Which would be a good symbol for them- a raven.)
(Bizarro idea- what if some of their leaders are actually humanoid ravens? Black tengu, perhaps?)
Raven, as it happens, is actually a German word. As is zepellin. Nice.
Oh, and they need robots. Or golems.
So what do these guys do? They build up their armies, biding their time, spreading lies in the courts of the other nations, conduct espionage, and try to steal magic items. And once the game starts to get underway, they start invading stuff. Looking good!