On the Leader question, let me interject a bit of Historiography here. . .
History, and especially history written since the 18th century, has always emphasized one of two characteristics: either it was a history of the Deeds of Great Men, or it was an attempt to identify, explain and analyze the Great Movements of people as a general mass. The former was almost the entirety of history written before the late 19th century, the latter has been the dominant 'strain' in the 20th century (and sometimes called 'Marxist' since Karl was one of the first 'analyzers' of social movements). The latter also has led to a lot of the transformation of historical writing by bringing in information/input from non-historical disciplines like archeology, climate studies, economics, sociology, and other sciences and near-sciences (or 'dismal sciences' in the case of economics). This has resulted in a host of 'sub-fields' such as Economic History, Social/Cultural History (which has come to nearly dominate American History) Environmental History, etc. Interestingly, 4X games almost by definition have to encompass many or most of these in some form, simply because of the '4X' definition of the game genre.
As usual with trends in any discipline, I think the best of modern history combines elements of both the basic forms: detailing the effect of the Great People in the context of their environment and the social, economic, environmental, political, etc strictures and pressures on it and them, like the reprint of Red Cloud's autobiography some years ago that put that Great Man firmly in the context of what was happening to the Lakotah and the Great Plains environment during his lifetime. No (Historical) Man is an Island, indeed!
We are now seeing these two historiographical 'trends' reflected very specifically in 4X historical games. Civ is firmly in the Great Man/Person camp: Great Leaders, Great People, personalized Governors, individual animated missionaries, traders, builders, settlers, archeologists, etc. Humankind, on the other hand, has no 'people' in it: no leaders, no generals, no builders, no settlers, no trade or religious units at all. The famous "Invisible Hand" of history governs a lot of what is happening: Factions/Civs Influence each other, your religion spreads (or fails to) as a result of Civics or Religious Tenets you adopt, and you change your entire Faction/Civilization each Age (usually) without regard to who is leading it or how (government/social structure) it is being led.
Frankly, I'd like to see each game combine the two basic trends more, but in the interest of competition in the game market and differentiating the games and the way they play, I can see why they would not: right now, at least as far as I've seen from the Pre-Release Humankind hints, they will play and 'feel' very different as games, and that's probably better for both game companies. And note that the third new competitor in this market, Old World, takes the Great Person right down to the individual family, ancestors and descendants by limiting the 4X to strictly Ancient - early Medieval Eras when families and dynasties were, in fact, paramount historically.
That doesn't mean there aren't individual game mechanics that could benefit in the games from shifting their emphasis: Humankind's tactical lay-out battles scream for modifiers from the effect of a Great (or not-so-great) General, while , IMHO, Civ's religious system would be much better implemented with more 'invisible' means of spreading and consequences beyond the gamer/government's control - which would also reduce what is now an entirely separate game of Missionary Invasion/Apostle Defense in too many games that simply distracts from everything else unless you are specifically trying for a Religious Victory.