What would be a French Civ without Napoleon? I've asked this question myself, and for this I designed Napoleon. In fact, two of them.
Napoleon I
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Leader Ability: Napoleonic Warfare - Ranged Class units' penalty to damaging cities is removed for Napoleon. Ranged class units gain +1 movement point.
Agenda: Napoleonic Code - Will try to have as advanced form of government with as many policy slots as possible. Dislikes Civs with government forms inferior to French government, respects those equal to him or better.
Everyone knows Napoleon Bonaparte. Brilliant military mind, great leader even today celebrated in France as national hero, son of a Corsican noble who began his career as an attendant of a military academy and peaked as the Emperor of the French who brought most of Europe under his knee, before being defeated in Russia by Tsar Alexander I and exiled on Elba, only to return to France, almost unbelievably regaining rule over France. The Napoleonic Code became the basis for most European constitutions.
I based Napoleon's ability purely on his genial use of artillery, represented in game by field cannons rather than bombards. All the way from Siege of Toulon to Waterloo, Napoleon was known for quick movement of his cannons and devastating use of them, being able to use them effectively for any range he wanted.
Napoleon III
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Leader Ability: Renovation of Paris - Every district grants +1 bonus housing to the city it was built in. Districts and buildings are built 25% faster in regular French cities and 50% faster in French capital. French capital gains 20% production towards wonders.
Agenda: Bonapartism - Will try to build a loyal society enforced by powerful armies. Will respect leaders who do not threaten his empire with greater armies near his Empire. Will dislike leaders who threaten France with large armies.
Living in shadow of his uncle, Napoleon III is often forgotten when talking about French history. And if he's mentioned, mostly his failure in French-Prussian War is remembered. However, while Napoleon III's external policy was unsuccessful (even though he successfully helped the Ottoman Empire against Nicholas I in the Crimean War, and managed to greatly expand French colonial holdings), his internal achievements are rather impressive. The renovation of Paris ended the era of city with narrow medieval streets, and turned it into the exquisite city of wide boulevards we know today, and generally worked on infrastructure and cities of his French Empire to become modern. This also included great expansion of French railroads. He managed to hold both French Royalists and Republicans at bay, keeping internal peace and stability in France until his capture in Battle of Sedan. The era of the Second French Empire was also an era of immense economic growth.
His ability reflects his internal successes in France. I wanted to call his ability "Franco-Prussian War" first, but naming it after his greatest failure felt strange, so I went with Bonapartism. It reflects his grip over France, and his tensions with Prussia.