I'd like the source code so I could look at certain things...
A lot of the modifications I want to do are:
Change to hex based map
Enlarge the scale (possible maps around 1480x2360, but with graphic tiles 1/4 of what they are now)
Adding terrain types
Increasing the amount of simultaneous civ's as well as the amount of total Civ's
Add the commodity system from Colonization, and change the food/trade/prodcuction system.
Change unit production similar to Colonization (equipping people with guns makes military units)
Add a bridge unit (Basically, a city improvement out of a city)
Get rid of current method of building TI's (My idea is a combo of PW and Workers... You have a labor pool, a resource pool, and you select the area(s) where you require major improvements. Silly little things like farms, irrigation, and mines will be micromanaged, Only Major improvements will require this method.)
Change the combat system and unit movement (to make it more realistic without harming the gameplay, I have created a model that takes away the traditional way of movement, and replaces it with an order system where you dispatch units, and then they carry out their orders, then return)
Change the unit properties (units will be taken directly out of the population pool, an army of 10,000 men deducts 10,000 men from the population pool of a city, an explorer group of 50 men, deducts 50 men, etc)
Change the population/city size to reflect actual population. No more 1=10,000, 2=30,000, 3=60,000.
Change population growth model and effectively eliminate ICS. I know this will make a lot of people happy. ICS will be somewhat eliminated with the population/city size thing, but with this new model, things like percentage urban population, population density, health, food, religion, and technology will all effect the equation.
Change the overall view of the map and gameplay environment. (Before writing/mapmaking, you simply see a raw sketch of the area. Then the more educated your people get, the maps will reflect that. The way I envision it now is -- raw sketch, Pharaoh[tm] type map, detail artist's rendition, exact topographical map, and finally satelite photos). Actual villages/towns/cities will have a more detailed, AOE/simcity type view with little moving guys and all. The hierarchy of the view will be like this -- town/small region(cities,roads, detailed terrain)/ large region(cities as seen from a higher altitude, roads, general terrain)/ world view (political, or satelite type view, with multiple zoom levels)
Production will be partly micromanaged, but major improvements will be player controlled. Also, simultaneous production will be possible in a labor screen that will have labor pool, resources, money, etc.
An evolving culture/language model.(this is too complex to list here, basically it solves the whole *yourcivhere*'s culture stays the same for 4500 years?)
On the highest difficulty level, I'm adding a ruler, revolution feature. This means that once your family's rule is overthrown, or assassinated, the game ends. This might seem stupid but it is actually fun. It does allow for changing government types where the player relinqueshes some control of the game to the AI. In effect, your people have their own AI. If you are too oppressive of a ruler, then you will be overthrown.
Politics model. As your civilization grows, you will require governors, mayors, and military leaders to manage the people. They each have their own little wants, needs, and strengths and weaknesses. You can also put leaders to death who defy you.
And finally, a morale, loyalty system for the military.
I have many other changes that I'm working on, but these are the only ones I actually have workable code for.
I'm hoping to put this together for a version of Civ 3, I have a playable version of Civ 2 with some of my changes, but it needs lots of work still.
If Firaxis releases the source code, I could easily put out a mod, similar to Counter-Strike for Half-Life. Basically, it not be a stand-alone game, you will need Civ 3 to run it.
I do have the FreeCiv, but there isn't much documentation. I've spent countless hours searching and searching through so many lines of code to find certain functions/algorithms and implement changes. And with my changes, the Freeciv version is going to be very large once I add graphics, sound, and animations.
Hopefully they will make a Civ 3 version of Freeciv. But I doubt many people will get exposed to my game because it will be so large without being directly compatible with Civ 3.
And getting a game pitched to a company is especially tough, I'm not even going to try unless I can get it polished, which with my team of 2 people, will take about 5 years, LOL.
It seems Firaxis doesn't even have the marbles to answer my question. If they aren't going to release the source code to any of the Civ games ever, the least they could do is tell me why. But I think it would be bad business to say, "Because we're greedy!"
),just use your imagination and play a hole lot of civ(1,2,Ctp,SMAC,...) 
.)
