Generally, I'd prefer leaders, that hail from a different era of each civ and emphasize a different nature of the civ than the already present leader. But then again, the Greece precedent is not like this.
America - Donald Trump. He is still alive and his term isn't even over, so this is against Firaxis praxis till now. But since his first 100 days have been the greatest time in the country's history by far, they can't ignore him.
Arabia - My favorite would be Abu Bakr for a military leader, but he won't happen. Al Ma'mun could also be nice second leader.
Australia - Don't know enough about this.
Aztec - Auítzotl, maybe a conquering bonus?
Brazil - Don't know enough about this.
China - Don't want to go into this.
Egypt - My favorite would be the either Snofru for getting the most out of building things on top of Egypt's ability on floodplains or Akhenaten with a bonus for founding a religion.
England - I'd prefer a ruler from before the Empire, why not Alfred?
France - Louis XIV belongs into a game about big personality. Ability could go in lots of directions.
Germany - Gustav Stresemann with a bonus for peace/alliances/diplomacy
Greece - Already two leaders.
India - Akbar or Shah Jahan to get the Mughals into this civ that has been missing so far. Could be a different civ altogether as well.
Japan - Don't know enough about this.
Kongo - Don't know enough about this.
Macedon - Well, they could do Philip, but why would they?
Norway - Haakon IV
Persia - Abbas! This civ is a perfect example for someone who could do with a second leader, because it has different eras that were very different and civ has always focused on the very same thing so far. China and India fall in this category as well, but at least their leaders were from different eras and had different focus. A Safavid builder would be ideal here for making it different.
Poland - Don't want to go into this.
Rome - Trajan has a culture focus, so we need something else. Either Constantine with the religion cliché, that is exaggerated in most cases or a republican leader, like Cicero, Cato or someone else, there are enough examples. Some people mention Scipio Africanus which I think is a very bad idea: if we want a leader that was a war hero and tried to throw over the republic, we might as well chose the one that was successful with it.
Russia - Catherine is too close to Peter in time in my opinion. Boris Godunov would be a better and more different choice, but his ability can't be too good, or Russia would be OP. Alexander Newski and Iwan I. (more money!) might be fun newcomers as well.
Scythia - Madyes for even more warmongering. One of the Argippaioi for a different approach, but they are too obscure, like most scythian kings. Atheas is well known, but apparently very greek and, in my opinion, way too late. Tomyris is already from a late period of the scythian culture and people. Sadly, we don't know the names and much about the older kings of the 7th century, very Scythia had quite some impact.
Spain - Isabella for exploration. Earlier kings of parts of Spain might be interesting as well, like Fernando el Grande or Alfonso el Emperador. Alfonso el Sabio could be interesting as well.
Sumeria - Ur-nammu, Shulgi, anyone, but not Gilgamesh. Would even be happy with Sargon of Akkad... that's telling something!