Picture inspiration: Grenadier of Napoleon's Old Guard
I never really understood why literally hundreds of years of history was almost completely ignored by fantasy writers, but I think I've narrowed it down to one thing: weapons. Writing battles or fight scenes where the characters are armed with single shot firearms (which have to be reloaded after every shot) is a bit tedious and not as exciting as say, a sword fight.
Still, there are rich veins worth mining. Every kingdom, empire, principality, and grand duchy had spymasters, hidden blades, and other ways to manipulate and hinder their rivals. Privateering was also hot, as was small wars between small little city-states. I have no idea what kind of story I'd write, but I'm leaning towards a story based on the Italian Wars of the 15th and 16th centuries.
Plus, I just really like old European military uniforms.