You do you! I do enjoy making designs, hence my modding repertoire. But you made this thread with the rather provocative title of "Why ask for an Italy civ when you can reasonably ask for two Italian based civs?", so I am going to respond giving my reasoning for why I think two Italian civs are completely superfluous.
To be clear, on a theoretical level, this is all fine. I have no problems having separate Athens and Spartan Civs, separate Kingdom of Sicily and Republic of Venice civs. You could make separate Calakmul and Tikal civs too.
There IS a very clear argument to be made that they all should be separate. Back in Civ 5, I made a Tuscany civ, and helped some friends make a separate Milan civ. We also split Greece up into Athens, Sparta, Pergamon and Macedon. We also broke Egypt into half a dozen pieces.
I'm only arguing against this on a "should Firaxis actually ever do this" level. There are limited resources, and if they ever decided "You know, we need Rome AND Venice, AND Piedmont-Sardinia" in the same game while Africa the Americas remain as sparse as they currently are, I'm gonna cry Eurocentrism and possibly boycott the thing out of principle.
On the other hand if the next Civ game somehow has Siam, the Khmer, Burma, Indonesia, the Philippines, Mali, the Zulu, Kongo, Benin, the Ashanti, Kilwa, Great Zimbabwe, the Hausa, Morocco, the Chola, the Mughals, the Mauryans, the Haudenosaunee, the Haida, the Shoshone, the Cree, and the Navajo already, then go ahead. You seem to acknowledge that those regions could do with more attention, so perhaps we aren’t disagreeing at all here.
In a game where Germany is led by a Holy Roman Emperor, but has a U-Boat as a unique unit, where Eleanor of Aquitaine and Victoria both rule England (about 700 years difference), where there China's Unique Unit (ca. 1350 AD), and their Leader (ca. 210 BC) is separated by about 1500 years, this is not a valid point.
To be frank: there's nothing special about intra-Italian politics. It's a bunch of squabbling city-states with related cultures, that we consider one group today because there's a unified state. No it doesn't quite make sense for Lorenzo de Medici to found and rule Palermo, or for Enrico Dandolo to be controlling Genoa. But neither does it make sense for Pericles to rule Sparta when the man is known for controlling the anti-Sparta faction during a war where the entire Greek world was basically split between Athens and Sparta.
The Mayans are probably an excellent comparison (and its politics during the Classical period HAS been compared to that of Italy). Multiple squabbling states. Varying degrees of influence from outside cultures. Multiple Mayan languages that are not necessarily mutually intelligible. From the cities I mentioned for instance Tikal and Calakmul were intense rivals during the Classic Period (250–900 AD). Tikal's culture and architecture was influenced by foreign invaders after they were conquered by Teotihuacan who installed a new dynasty there, whereas Calakmul's lineage was native Mayan, tracing its roots back to El Mirador. Lady Six Sky herself ruled Naranjo, which was a smaller City-State that was part of the Calakmul side of the alliance. Strictly speaking, it makes no sense for her to rule Tikal. She may not have identified with them or their culture. She still can found and rule Tikal.
Besides, like it or not, we ultimately do have an unified Italy. If you get down to it, we never did have a unified Mayan state, or a unified Cree state, or a unified Maori state. Meanwhile Gran Colombia stuck around for 12 years and broke into a million pieces.
I'd like to point out saying "And France, England, the HRE united long enough to fight the Third Crusade" doesn't really mean we should have a united Christendom civ
India is a mess, frankly. It covers millennia, two language families (at least all the major languages of Italy are from the same family), and too many religions. And then the Muslims attacked and made everything even more confusing (Mughal civ when TBH).
I don’t know much about Indian history frankly, I just know enough to be able to say they certainly need more love than they’ve been given.
Frankly, I think Scotland was a Eurocentric choice. Meanwhile France and Gaul is probably even more distant than Rome and Italy.