If I was to write a "civilisation creator editor", I'd first make a "clone an existing civ" tool (that duplicated all of the entries for an existing civ).
Using the modular XML mods, this wouldn't be that hard, because you'd just be creating a new civ.
The hard part would be validation. I'd have to reverse engineer what kinds of validation that the game does.
As a guess -- getting a crappy civ cloner/editor out the door might take 8 hours. Getting a polished one out the door would take many, many weeks -- really, the sky is the limit (do you include tools to edit leaderheads? Edit the look of units? How about merely the position and actions of units? (for UUs) etc).
Remember, to make a civ, you need to be able to:
1> Build a unique unit or two. These units should look different, or at least have the option. So you need a full-fledged unit editor! Or, you need a unit library tool and a seperate editor that creates units.
2> Build a unique building or two. These buildings should look different, or at least have the option.
3> Build at least 1, if not more, leaders for that civ. The leaderheads should possibly look different -- so you need at the very least a leaderhead loader (ie, some kind of leaderhead library management tool).
4> Set the attitudes of that leader. Remember, we want intuitive -- so you need descriptions of what each and every parameter does!
5> Set the properties of each such leader.
6> Easy stuff, like lists of cities, starting techs, etc.
7> You need to add civopedia entries for the unique units, unique buildings, leaders and civilisations
8> Don't forget stuff like preferred religion -- and a full description of what it does! Player colour, initial civics (for mods!), sounds, etc.
9> Setting art type styles. For a good editor, you need the ability to load libraries of art types, and pick them, and ensure that all units for your civilisation have art (and buildings).
10> After you did all that, you'd still have to rely on the game engine itself to validate that the generated XML is valid for a given mod of Civ 4.
These aren't trivial tasks. You need to replicate most of the civ4 config engine so you can do things like "make a civilization compatible with mod X", which can include XML files of various styles, possibly modified schemas (!), modularly loaded XML files, etc.
Unless you roll the leaderhead/unit/building/civopedia editor into the system, you need to have some kind of library management utility. See, UUs for civilsations aren't valid in a mod until the civilisation is there -- and the civilisation isn't valid until the UU is there. This is a chicken and egg problem for validation.
If we want polish, then you need to solve that chicken and egg problem. Which basically means you have to have "civ4 fragment libraries", like UUs that don't have a civilisation attached (and maybe not a tech or a base unit), and a library management system for them so you can "grab one" and use it elsewhere.
At this point, we are talking about building a suite of tools. And as the people you are making it for are command-line phobic, they all have to be GUI tools. And as the people you are making it for are scared by errors that don't happen immediately, you have to make everything self-diagnosing, idiot-proof and/or generate errors immediately, and not deferred.
So sure, such a GUI toolkit could be done. But civ4's engine is so much more flexible than most other games of the kind that there is a lot of work that would need to be done in order to make it "good enough". And even after all of this, you'll remember my mention of "mods with different schemas" right?
Someone can mod civ4 so that leaders have some new property. How that new property works is not anything that your editor will have a clue about, because how it works will be described in python or C++ and not in XML.
And I went and glossed over the entire "oh, create a leaderhead editor". People making leaderheads tend to spend multiple 100$ on the program that edits the graphics files, or pirate it -- neither of which is practical for someone shipping an official game editor.
But don't trust me. Try it!
