Would really like to see an expansion that focuses on prolonging the Early game which is the most fun.
It would be a Dawn of Civilization expansion that adds new techs at the beginning of the tree. Starting civs begin at a hunter gather level and would have to research "Fire" and Basic tools. Agriculture wouldn't be a starting tech so you would have to research it. Palace would also have to be something you build rather than given it at the start of the game. No making settlers until you research leadership. There would be a couple of new units that are weaker than warriors including a stone age archer with one hex range.
Interesting new complexities is that resources are hidden until the tech for them is researched. No founding a city surrounded by gold and gems until you research mining. Dyes and incense wouldn't be revealed until calendar. Fish, Crabs, and Pearls would be hidden by a new pre sailing tech called "fishing." New Boat making and wood working tech would provide canoes in the era before Triremes. Also maybe a pre-wheel road called a "trail" could reduce terrain cost in forest, jungle, and hills.
Would also be lots of new Wonders such as Gobekli Teke, Puma Punku, and Nan Madol.
New units would also be added later in the game to fill out more logical upgrade paths.
Lancer should become a direct upgrade to the Knight.
All civs would get horse archers which upgrade into Cavalry as a mounted gun powder ranged unit.
Pikemen should upgrade to Bayonet muskets in the Renaissance era.
New unit Grenadier (Grenade thrower) would provide a late Renaissance era upgrade to the crossbow.
This way the upgrade paths are more distinct.
(Mounted Melee) Horseman > Knight > Lancer > WW1 tank
(Mounted ranged) Chariot Archer > Horse Archer > Cavalry > Gatling gun
(Pole Melee) Spearmen > Pikemen > Bayonets > Anti-tank
(Basic Melee) Warrior > Swordsmen > Long Swordsmen > Musketmen > Riflemen > Infantry
(Basic ranged) Stone age archer > Archer > Composite bowmen > Crossbowmen > Grenadier > Machine gun
Revamping the techs in the enlightenment era would also be a good idea.