I was thinking about having a system, or maybe an empire in my game that uses rocket ships with jump drives. You know, the typical 1950s rocket ship that looked so splendid back then.
Other ideas I had were worlds that uses diesel walkers, highly-functional steam technology, or somehow created technology based on bronze and something akin to the Antykitherian mechanism. Those ideas most likely don't make sense, but they would be cool to meet in game.
I currenty am working on a long-term project of reading as many old pulp magazines as possible, to get a nice overview of the history of the genre (unfortunately I don't have as much time as I want and some of this stuff is hoary). A lot of stuff is not quite as sensible as one would think. In a lot of these old stories people had just a bad grasp of how science actually worked, no matter how proud they were to be so much better than all those peasants that don't know anything about science.