I'm pretty sure they will add civilisations/leaders not for their relevance in History, but for the gameplay they can add in the game. I think that's why we have Catherine de Medici: for the Spy and Diplomatic Visibility features. I think if they add other leaders and other civilizasions, that will be for the same reason.
If they add a new leader for France, let's say Napoleon, he must be more than just an other warmonger. They will make some research and try to explore new concept, like the dechristianization of France during the French Revolution, and we will have that kind of things:
Napoléon:
Cannot create a religion. Can purchase religionless religious units with Gold to reduce religious pressure in his cities and engage religious combat. Holy Site adjacency give Production instead of Faith, Shrine and Temple give Science instead of Faith and Great Prophet Points is converted in Great General Points. +7 Combat Strength against religious civilisation.
You have it: an anti-religion warmonger civilization. That also work to create civilisation efficient where all other can't, like settling in toundra or desert tiles, using Relics effectively or make tall cities.
I think the good question isn't: What civilisation in History deserve to be in the game?, but more: What civilisation is unique that can be translated into new mechanics in the game?