Some suggestions I've put together:
-Make Manhattan Project worth making
-Give the Khmer, Carthagians, Celtic (I mean come on, their UB makes the unique unit no different than the normal unit), Danish, French, Greeks (Well, their UU is pretty good as it is. I'd just like to see it renamed, and maybe instead of getting +100% vs chariots, it should get 50 against archers and chariots)
-Make it so castles don't become obsolete till a later tech, or make it so they become available earlier.
-Give the French a better UB.
-Change around traits to be more realistic. Firaxis gave some leaders odd trait combos due to their rule of no doubling up. Since we don't have that rule, we can go for realism instead.
Also, some trait suggestions:
-Productive: +1 hammer on tiles with at least 2 hammers.
-Agricultural: +1 food on tiles with at least 2 food.
-Secretive: +4 espionage per city, faster production of Security Beaureu and Intelligence Agency
-Seafaring: All naval units start with Seafaring 2, faster production of Lighthouse, Harbor, and Drydock
It'd be kinda neat playing as a leader known for having a great navy and getting the bonuses that make your navy just as great. I guess they gave those leaders Financial instead because Fin means you'll be building along the coast.
And my suggestions to improve the earlier mentioned UUs:
-Khmer: Maybe increase its strength by 1? Or give it a bonus against other mounted units.
-Carthage: Give it a city attack bonus.
-Celtic: Replace Guerilla 1 with Guerilla 2 or City Raider 1.
-Danish: Maybe give it Flanking 1 as well.
-France: I'd suggest maybe giving them Pinch. For their UB, I'd suggest just going with an entirely new UB. I'll try to think of a better building to represent France. I don't think I've ever seen so much hate for a UB, though. I guess because artist's aren't very useful, but the building it replaces is, so you're kinda stuck with an artist.
-Greece: Like I said, maybe changing +100% vs chariots to +50% vs archers and chariots. ...and please rename them. I'm not sure what Firaxis was thinking.
And of course changing the Immortal to either a light footman or a footman instead of a chariot.
Edit: Thought of a couple of ideas for a replacement French UB. Either a Chateau that replaces the Castle and serves as a culture booster in exchange for removing the espionage boost but lasts for longer than a normal castle, or a Bistro that replaces the grocer and offers either a happiness or culture booster.