A couple of ideas if Firaxis lets this happen:
1) If there are going to be more than the allowed leaderheads, we should have one mod with a configureation program, which would let you add or remove leaderheads, graphics mods, interface mods, units & their resources, terrain, buildings, etc.
If certain features go together e.g. a resource goes with a certain unit and building, then those two would be tied together, so if one is added, the other is too. This would make it very professional. Along the same lines as the Mods menu and UMOD installer in Unreal Tournament.
2) Dont take on too much work. Write a small list of major features you really want. Finish all those, then move on to all the good ideas and small features that are suggested. If you start work on the small things first, the major features will be lacking, and the major features are what sells. That is the mistake that Firaxis made, at least I think. They didn't finish the MP part of PTW before they started adding units/countries etc.
Good luck! I hope it works. I would love to help, but sadly I lack the skill to do anything worthwhile. If you need any laborious and repetetive work done, e.g. adding 200 civilopaedia entries, I could help with that.