I want tinkering with the graphics to be easy.
I agree. I think allowing the importation of Civ IV or Civ III graphics would allow some flexibility for those of us that are not artists to add additional leaders or units. The graphics may be imperfect for the game (ie: leaderheads would'nt take up the whole screen, Civ IV units show 3 soldiers whilst Civ V may show more), but it provides more options, especially for early adopters. Besides, a good quality Ataturk from Civ IV on the side of the screen is always going to be better than a hastily done full-screen Kublai Khan re-skin dressed in a suit.
Anything to make the artists and programmers jobs easier though, will hasten the proliferation of mods available on these fora for download. A ' default' graphic used in place of missing/ill-defined graphics would be a life-saver. Crashes to desktop due to missing art is unnecessary. It would need only be one of each
type. Eg: a 'default' leaderhead wearing a "Missing Art" T-shirt, A "clown car" with comical cloud of smoke to use in place of
any missing unit art, a bitumen terrain graphic that replaces
all missing terrain art, a pic of a scarecrow (or a sign that says "fix me") for missing improvements, a "question mark" button etc. They may look silly, but they would allow you to keep playing (they wouldn't affect the text, which would still work as normal) and would stand out like a sore toe to graphic designers testing their work (or for downloaders who could easily file an accurate bug report).
Missing XML could be similarly fixed in some cases. Having default values would be better than crashes. An example at work would be a default terrain type to use when XML points to something that isn't defined elsewhere. So if a mod tries to call TERRAIN_PEAT_BOG, but haven't defined it in XML, then TERRAIN_DEFAULT would be used, which might be the 'bitumen' mentioned earlier, using the default yield, and allowing only the 'default' improvement (eg: scarecrow - with its own default yield, and which would also be used in place of any ill-defined improvements a mod calls).