I don’t care what the calendar says: 98 degrees is an evocation of summer if there ever was one. Close enough at least to mention some vacation reading selections that could also provide some rpg inspiration.
Leviathan, and its sequel Behemoth (and Goliath on the way) by Scott Westerfield imagine a quite different World War I where the Entente Powers are Darwinists (utlizing “fabricated” animals as technology) and the Triple Alliance are Clankers utilizing mechanical technology far advanced of our real history. I suppose the setting might be called steampunk, but the mechanical technology is firmly hydrocarbon-powered, and the biotech adds a new angle. The series follows Deryn Sharp--a Scottish girl masquerading as a boy so she can join the British Air Service, who winds up serving on the bioengineered airship, Leviathan, and Aleksander of Hapsburg--secret heir to Austro-Hungarian Empire, currently being hunted by his country’s German allies.
There is, of course, the hint of possible romance between the two, and conveniently the adults are often out of the way so our teen protagonists can save the day (these are YA novels), but there's plenty of action--and beyond that--there’s a lot of interesting worldbuilding and plenty of neat alternate tech for any sort of rpg. Then there’s the great illustrations by Keith Thompson to really inspire:
The Half-Made World by Felix Gilman is also about a Great War, but this one is in a fantasy (rather than science fiction) context. The West--a part of the world only becoming “stable” as it's settled by humanity--has become a battleground between two groups controlled by inhuman powers. The agents of the Gun are notorious outlaws, given superhuman abilities by the demons inhabiting the firearms they carry. The people of the Line live regimented, industrial lives in the service of 28 sentient Engines. Caught in between are the mass of unaligned humanity, and the mysterious and powerful Folk--the original, nonhuman inhabitants of the West. The knowledge that there is a weapon--a thing of the Folk--that could end the war sets in motion a race to retreive the one, brain-damaged man that may know its whereabouts. This man, an aging general, and his hapless doctor get caught between the forces of Gun and Line.
One caveat: there's a sequel coming, so it's not "done in one." Don't let that dissuade you. Gilman’s world has a lot of great ideas to steal for an rpg setting, and gives great example of non-medieval secondary world fantasy to stand beside those of Mieville, VanderMeer, and King.
14 hours ago