The Romulans were quite developed on the shows. We saw their culture, politics, homeworld, intrigue, spies and more.
That's all surface stuff compared with the development other major aliens got. I'd like to see someone finally dig down into the Romulans and tell us what's unique about them.
Vulcans are all about logic but that's just masking their violently emotional true selves; Klingons are warlike and natter on about honor (hypocritically in many cases); the Cardassians are "bad" but strangely likeable probably because of their unique combination of intelligence, charm and self-destructive egotistical delusion. The Founders are paranoid whackjobs who invented a society at war with itself (Vortas and Jem'hadar are deliberately designed to be at odds) because their goal above all is to ensure their minions never unite and rebel - a society that is both sick and functional.
The key to the Romulans is that they must have the same problem the Vulcans do. They are too violently emotional to have a coherent society unless they come up with some solution. They rejected the Vulcan solution of fetishizing logic - which I can sympathize with, if they rejected it for being hypocritical - so what's the alternative they've hit upon instead?