I like Civ 4 in terms of civilization building, however combat is the most serious issue that still plagues the game.
I have edited my own version to add each unit with terrain attacks and defence bonuses, an example is that chariots are better attacking units in grassland that on hills. Due to this it means that you have to think not only about the units abilities but also the terrain there both on.
A another thing ive added is era based bonus's on units, for example armoured units have a huge bonus vs melee, and like wise melee units have a negative bonus vs armoured. This means that tanks in the game can pound 36 swordman units and not even get a scratch.
I ve also added a addition civic option known as military, with Militarism, Imperialism, Chivalry, Mobilisation and Peacekeeping. They all have a effect on your military such as Chivalry means you can draft five units per turn, Mobilisation is sweet for war as it grants 100 percent military production however it stifles the economy as each unit has additional support cost, meaning less research and culture rates.
I would like to add a new civic much like RFC expansion that helped civs build colonizing empires without the crippling maintenance cost. I played a game on huge map and found the cost so extortionate that i couldnt possible acheive a conquest victory even though my research, culture and espionage were all at 0%.
I would also like to put in additional features such as civil wars, like a nationality, tax, religious, political, independence or war weariness agenda between citizens that means civil war, however the war would only go on for a select period, such as a minimum of ten turns and a maximum of thirty, and who ever had the advantage at the end (killed the most, took the most cities, damaged the most improvements) would win.
I also like the idea of barbarian states becoming civs of there own if left, for example if on a fractal world and a state started up on a separate island it could flourish (however slowly at first) into a distinct seperate civ.
I also tweaked my version so that units are much easier to build. I can understand the great wall taking a thousand years to build, but it takes also a hundred years to arm a bunch of neoanderthals with wooden sticks.
I like Civ 4 however i think there is still much to do.