I foresee that leading to as bad of results as the changing clothes in Civ3. Frankly I don't want nationalist nonsense like "Matthias Corvinus leads the Sumerians in the Middle Ages!" or "Saddam Hussein leads Babylon in the Modern Age!" or "Benito Juarez leads the Aztecs in the Modern Age!"
True, but here is also Potential to a Rise and Fall of Nations mechanism, where Civs if aren't stable (would require a new national Yield, like Stability in Humankind) they may fall as that Civ but rise as another (from a Pool of suitable Civs), merge with neighboring Civs with low stability and creating a cross-calture Civ (the Civ Name and Leader will be either history/legend (we already have Gilgamesh and Gorgo in the Civ Games) inspired or just improvised ( from a preset of names/leaders - the human Player can choose the name and leader)) or maybe flip to a neighbooring Civ that puts enough Loyalty Pressure for the first to flip to it rather than founding a new Civ (yeah, I'm suggesting an improved Loyalty Mechanism taken one (or more) step(s) further). But it has to be really hard for a Civ to keep itself stable to not fall, which would lead to a
true Civ
standing the test of Time (I never got this feeling in the Civ Games, No one can, the Civs/Leaders (apart from Civ3 or was it 4 where you could switch leaders durring a game?) are the same for the whole Game).
I think this would make for an interesting Gameplay, but also would make it possible to introduce new Civs that don't have a high chance to be introduced into the Game with its current model, and instead of searching for leaders of a Civ for each era, we could have 1 leader for that specific Civ only. Like for example we could have a Game where you could start as the Numidians, then may later turn into The Berbers (if you didn't merge with Rom and rose as . . . Num( lol ??)) and finish the Game as Morocco.
Admitted, designing all those Civs will obviously take much more time than with the current Civs model, including choosing a Unique for each Civ, but I would rather sacrifice the 3D Leader models for just 2D Portraits of the Leaders to have this kind of mechanism (I would do anything for this kind of model), which I think is much better than how Humankind is doing it (the main reason why I'm not eager of getting that Game, unless it will be moddable).
So yeah, this way Saddam Hussein coold lead Irak instead of Babylon.