An effort to make a second distict leader for each 1-leader civ would be great, and when I say "distict," I mean high-quality, unique-looking leaderhead (such as the Hitler, Hirohito, Massina, Sadam, Basil II, etc) that you have to look at hard for a few seconds to figure out where it came from. Anyway, here are some suggestions, OK more of a wish-list:
Persia: Xerxes (he was in Civ III, let's get him back)
Carthage: Dido (would be awesome, but would probably be a big challenge as she would require a classical-looking female retex, and she's not as historical as perhaps other Carthaginian leaders).
Arabs: Nasser (yes, technically he was Egyptian, but he tried to create the Pan-Arab league), also could use Saddam (even though he's technically more Babylonian).
Greece: ? (there are some nice Classical male retextures out there, or a new one seems possible by basically changing the hair color, adding some wiskers or some such thing, and slapping a new breastplate texture in place).
Aztec: ? (no clue)
Inca: ? (no clue)
Japan: Hirohito (already done and looks marvelous), also Japan really should have a post-WWII financial leader available.
Mali: ? (no clue)
Spain: Charles Martel (warlike, maybe something like the chain-mail retex of Augustus. There are plenty of other Spanish options out there too, and I think it's a shame that a major European civ only has 1 leaderhead).
Celts: Boudica (some attempts have been made, but none really strikes me as distinct enough yet)
Korea: Kim Jong Il (maybe could be done with Mou? I don't really know of many other Korean leaders.)
Ottoman: Attaturk (Civ Gold put him in, but while he's a great choice, he just doesn't look at all like Attaturk.)
Viking: ? (I know that there's at least one out there made from Peter, but he could use a bit more ruggedness, at least some mail or furs or something.)
Zulu: ? (I'm not a great Zulu historian.)
OTHER MODDED CIVS:
Israel: Joshua or Macabee (retexture of Hannibal perhaps)
Byzantium: Constantine (perhaps a Julius tweak?)
Again, this is simply a wish-list of sorts, my ideas for how to even out the civ leaders. If anybody is wanting to do a leaderhead and has no idea who, I would invite you to please pick one of these, and if all get done, could somebody please compile them into a component or modpack? I know it's a long-shot, but thanks for reading.