sorry for necro of post but since this is exactly what I was going to "wish for christmas" I will second it.
As it is now, unless changed by mods, all nations always have the same leaders and the same UUs,UBs and traits.
This limits the players choices in game actually.
Take England or Ottomans for example, while they are not completely useless on a "all-land" map, their mainly Sea oriented traits or better their absence make you think twice before picking them up.
Its no biggie, with BNW and all the other DLCs combined, we have more than enough alternatives but if there is another expansion planned than I would surely wish for at least another leader who not only changes the "flavor" of the diplomatic screen but also comes with his own UUs and/or UBs and traits.
America for example could get Lincoln as an alternative Leader, a different Ironclad as a UU (the Monitor) or how about a UU Trade Ship (the Mississippi Liner, bonus gold from onboard gambling

) and a different trait (example "Emancipation Proclamation" - less unhappiness in occuppied cities) or something like that.
There are numerous advantages having multiple leaders. Since changing UUs/UBs and traits basically changes the whole Civ, we would double, triple or more the Civs that are already in the game. Beside that, every new game with random civs you would be excited to see who you are up against this time. Personalities of different leaders could be totally different meaning that some will live and love peace while others knock on your doors with a big stick and as already mentioned, I would love a diplomacy screen with Stalin in full Civ 5 grandeur

(since the other guy has even less than a chance of appearing at all, I better not mention him).
All in all, it would be in any way a welcome addition and I doubt anyone would be against it.