I'm considering changing the Expansive trait, myself. Can you tell me which file I need to change?
The forums on creation and customization are the best, but I can state a couple of basic ideas I had and tried:
First and foremost, never change anything without copying the the Civ4\BeyondtheSword\assets folder. Save your copy as something like BackupAssets. That way if you make the game unplayable by goofing around, you can delete your project and change the name from BackupAssets to Assets and the game will run the unchanged assets now.
Open files in notepad to edit
It's basically a matter of reading through the XML files and making decisions. The helpful files are Civ4\BeyondtheSword\assets\xml\units\civ4UnitInfos; Civ4\BeyondtheSword\assets\xml\buildings\civ4buildingInfos; and C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Beyond the Sword\Assets\XML\technologies\civ4techinfos.
These allow to modify units, buildings, and techs as you want. If you know you want a certain effect, find something else that does what you want and make similiar changes to the unit/tech/building in question.
For example, I've long felt that Divine Right should grant a great prophet, because it was the only GP not acquired by a tech and it gives something good to beeline it (well, I love Versailles, but still).
So, I went into the technologies/civ4techinfo folder, and looked up music because I know music grants a great artist. About the 12th line down in the text I saw <FirstFreeUnitClass>UNITCLASS_ARTIST</FirstFreeUnitClass>. Ah! This is what I was looking for. This is what tells Music the first to earn it gets that artist. To confirm, I looked up Fusion, and found <FirstFreeUnitClass>UNITCLASS_ENGINEER</FirstFreeUnitClass> in the same place. So I knew the relevant thing to change now, so I found Divine Right, sure enough there was a line<FirstFreeUnitClass>NONE</FirstFreeUnitClass> and I changed that to UNITCLASS_PROPHET. And it worked.
So anyway, in short, if you want to add abilities look at units/techs/buildings that can do what you want (trade routes, extra commerce, etc.) and then make similar changes. In this way I've made the following tweaks:
Great generals can start golden ages (to give Imp civs some help).
Double production of stables (ger) and jail (mausoleum) for Imperialistic leaders.
Ballista elephants now do collateral damage (minor, only 2 units per attack up to 30 percent). Gives them a little something extra.
Castles (citadel) now obsolete with chemistry.
Divine Right grants great prophet to the one who researches it first (Islam gets a shrine that way, or maybe you save it for a GA).
Expansive is 50 percent worker (fast worker) production bonus, not 25.
Can't build Scotland Yard until you've researched printing press
Can't build Academy until you've researched literature
I'm not trying to change the game play, I think the game is well balanced. I'm just trying to tweak some balance and make all buildings/traits/units worthwhile if you want them. I'm thinking of making one more change to, you can't build nukes without a nuclear power plant in the city (I never build Nuclear plants, I usually have coal or hydro or Three gorges). Also, I may give Duns some spy points, because Guerrilla is pretty cool but seems lacking. The other weak UB, Citadel, I already took care of. I don't think Shale plants are weak at all because power without coal is useful and there is a small hammer bonus.
Anyway, read the creation/customization forums and they'll steer you better than I can. Just don't forget to save a copy of a the original assets folder, that's the key for not having to reinstall BTS if you get carried away.
EDIT: Gave Nuclear plants a free scientist specialist and gave Duns 10 percent espionage defense. I only have one more project and then I can play my retooled game ... A special forces unit for paratroopers/marines to evolve into. After that, I think we have a balanced game ...