One of the problems with space opera, particularly books like Ark Royal, is that almost all Great Powers are spacefaring powers by definition. There’s no easy way to represent the differences between – say – the Britain and France of the Napoleonic Wars . A land-based power in space opera would be easy to smash flat, so they are all naval powers. However …
Imagine a universe where many systems are linked together by wormholes (or warp points, whatever.) Maybe some of those wormholes are artificial. Space warfare is based around the warp points, as in the starfire universe … unless you are prepared to spend lots of money on giant FTL ships. On one hand, these ships are staggeringly expensive; on the other, they give you a degree of tactical flexibility no non-FTL ships can match.
How does that sound for a setting?