France. France by far and away deserves alternate leader the most, imo. Come on. Generally I think Firaxis did a good job with representing civs and leaders in this game - with di Medici being
notable exception.
And not only she is historically controversial choice, not only she is fairly unlikeable, she also has - at best - mediocre leader ability. Adding alternative would buff France.
I'd pay to replace her with Louis XIV, or Henry IV, or Philip II... Or Cardinal Richelieu. He technically wasn't the ruler of France but I'd overlook it just to see him, it'd be incredibly badass.
Also:
- king Alfred the Great for England
- Frederick the Great for Germany, I really want to see this fascinating character
- alternate Chinese emperor who looks like a warrior badass, not fat cartoon
- alternate Arabian sultan who looks like a warrior badass, not thin cartoon (okay, I'll be honest, I mostly want Saladin to look differently, I don't even need alternate leader

)
- I'd support Scipio Africanus to represent republican Rome
- FDR for America, if only to have two Roosevelts ruling USA, for joke/meme purposes. Also, leader in a wheelchair would be cool. Also he was good president etc

- some ancient, stoic pharaoh for Egypt, to contrast with expressive, feminine Cleopatra