I'll reply to the original post in-depth and try to cover what's been said afterwards as well.
Ok Leoreth, got a few suggestions and comments.
I think it would be great if the Carthage, Mayan, Greek, and Babylonian slots were replaced for better modern civs in the 600ad start (suggested civs Canada, Argentina, Australia, and Prussia).
I think that would be great too, which is why I'm working on the Italy prototype to figure it all out.
My personal opinion on the civs mentioned during this discussion:
Canada and Australia
I think those are best represented as English colonies. They got independent quite late in history, so their game would be short and their overall historical impact wasn't that decisive that it would warrant a reasonable set of UHV. No offense my dear Canadians and Australians, this is actually a good thing
Argentina, Peru, Colombia ...
Basically the same situation like above, but they have a little higher chance just because South America seems so constant currently.
Sumeria
Is represented by Babylonia in my eyes. For the timeline to fit, it would have a ridiculously short time to live and no place to share with Babylon (we even would get problems with the city distance).
Korea
Is among my favorites, actually. A one city challenge along the lines of Babylonia is definitely possible, and could be a lot of fun crammed in between China and Japan, whose interaction I often found too onedimensional. Throw in another civ for the fun
Poland
I am very much aware of Polands significancy for European history, especially in the Late Middle Ages until the 17th century, but again, there's no spare place. Germany is already severely crammed in in the west, and another civ in Europe would mess things up even more. Sadly
Germany, Prussia, HRE, Austria
Why, of course I waste four slots for them

In my eyes, Prussia is identical with (modern) Germany. After the unification, it made up 2/3 of the Empire, and many customs and symbols of Germany have been inherited by Prussia.
On the other hand, there will be a HRE that hopefully collapses before Prussia/Germany spawns (envisioned date: 1701). If its still alive at that point, or respawns later, it simply becomes Austria instead (in actual history, the Habsburgs tried to rescue a little bit of the HRE in continuing to carry the emperor title).
So it will be just like in BtS with two civs: HRE/Austria and Prussia/Germany.
Wien isn't a great historic choice for the Holy Roman Empire. A better choice would be Frankfort/Mainz--plus vienne was at the borders of the empire for a long time.
I've already explained some posts before why I think Wien fits the HRE. But I agree with you that it feels weird to spawn where at that time was only a marcher county, especially when the new flavour text talks about the East Frankian Empire.
So while Frankfurt was never the capital of any form of German state (though it came close in both 1848 and 1949), it may be a fitting capital, both historically and economically (ingame, that is).
Also-- what's up with the babylonian unique power's effect? I know that its Rhye's, but... you've changed others. Maybe the UP should be changed to have whatever capital city you choose to use be considered "Babylon".
--> I prefer to play as the Babylonians and any change to improve would be great. Maybe also make their unique building quicker to build? maybe make it a granary?
I think the UB is fine. The UP needs to be changed, but I think founding Babylon on the spot is an integral part of Babylonia's challenge. Anyone can move some tiles to get more production or food

Babylonia's, India's and France's UP all might get in flux the next days, though.